What follows are my initial impressions of what Infinity considered, at one time, the "... third-best speaker in the world." I don't think it's necessary to get into the lineage and intricacies of the Quantum series or the evolution of the Quantum 3, as this information is documented elsewhere by people much more knowledgeable than I. I will say that the Quantum 3 is a vastly scaled-down version of their erstwhile TOTL Quantum Line Source (QLS-1), and, while it uses all the same drivers, it uses less of them (save for the woofer and mid-bass coupler). The hallmarks of the Quantum series were the use of the then revolutionary EMIT tweeter (a planar tweeter), and the Infinity-Watkins "dual-drive" woofer. Further, the upper echelon of the Quantum series utilized phase-aligned driver placements, and simple 1st order crossover networks designed to maintain proper time alignment. In any event, these were supposed to be the proverbial bee's-knees among speakers, maintaining the performance of the QLS-1, but in a package and price that was just over half the cost. When I got this set home, I discovered, to my horror, that one of the mid-bass couplers was not operating. Knowing, as I did, that the L-pads in the crossover had seen much better days (and indeed, were more or less corroded beyond redemption), I bypassed the L-pad (as I had done for the EMITs in the other speaker), but found that the coupler was still not outputting any sound. Further investigation found that the driver did indeed have an open voice coil. However, I was undaunted and looked to another set of speakers I had on hand that used a very similar driver as a midrange unit. The other driver, unfortunately, was a 4Ω unit, as opposed to the 8Ω version used in the Quantum 3. I put in the call to the seller, knowing his propensity for having spare parts for everything he owns, to see if I could beg another mid-bass coupler from him. He did not have one though, but offered to make it right by reimbursing me for the cost of another, should I find one. Having not much left to loose, so to speak, I worked out how installing the 4Ω unit would affect the crossover points. Thankfully, Infinity made my job easier with the addition of many series resistors- what would have been a drop from 8Ω to 4Ω was going to be a more benign 13.5Ω down to 9.5Ω. This meant that the bandpass crossover would narrow its bandpass, but only by a little: from 200-600Hz to ~250Hz-550Hz. This, coupled with the gentle 6dB slope, meant the harm from changing the driver would be minimal. I mention this mostly because it I think it will have changed the performance (and thus my analysis) of the speakers, but perhaps not to a large degree. I will say that, when I had one speaker configured with the original (working) mid-bass coupler and the other with the substitute, the difference between them was not too great, but it was noticeable. I did replace the mid-bass couplers in both speakers with the ones I had on hand so as to maintain balance between speakers. So, on to the initial listening. As usual, what follows are just my first impressions, and my opinion is likely to change with further critical listening and the eventual replacement of the mid-bass couplers with the original, correct part. For the past year (more, actually) I had been using a set of IMF TLS-50's, which are the speakers by which the Quantum 3's are judged. Right off the bat, however, they seem to possess greater similarities than differences. Both offer extended low-frequency response, as well as adequate high-end extension, and both offer smooth, balanced midrange. Where the differences lie are in the details, and when comparing both directly, each holds an edge over the other in certain disciplines. The first noticeable difference is that the Quantum 3 seems to have some amount of coloration in the midrange. This has the effect, when compared to the TLS-50, of implying a more "boxy" sound. As of right now, I attribute this mostly to the substitute driver I'm using, as this coloration lies right in the range that it is operating in, and the sound is that of the mid-bass coupler not smoothly blending with the dome midrange driver. Aside from that, the midrange region is quite good, with lively, but not hard vocals and strings. The midrange is slightly more forward than it is with the IMF's, but this is most likely due to having to have the L-pads turned up all the way (or nearly so) in order for them to pass sound. Once the L-pads are replaced, and I am better able to dial in the sound, I'm certain that any issues of midrange forwardness will be resolved. As it is, it is only just noticeable and does not distract from listening. The high-end of the Quantum 3 and the TLS-50 are very much comparable, each possessing very smooth high-frequency reproduction. Where the Quantum 3 has the edge is in the very top-most air. This is a region that the TLS-50 can play into, but it does not seem to impart the same sense of effortless and infinite extension that the Quantum 3 does. Where the TLS-50 holds its own, and indeed outperforms the Quantum 3 (at least in my room) is in giving a sense of space. The Quantum 3 does provide a fairly good L-R soundstage, better than I anticipated given that they are not mirror imaged, it does lack the sort of depth that I've become accustomed to. In that regard, though, it is in good company with other very highly regarded speakers. It's not that they do not give a sense of depth, but rather that I have yet to hear a speaker (aside from properly setup Maggies) that can compete with the TLS-50 in that regard. Further, the dipole, rear-facing tweeter of the Quantum 3 could be a liability in my room, as one just fired down the length of the basement, while the other hits a wall not more than a foot behind it. Bass performance is where the two systems are most different, and indeed, each covers a performance metric better than the other. Bass from the TLS-50 is exceptionally deep, sonorous, and tuneful. It sounds best with pipe organ, bowed or plucked double-bass, and synthesized ELF bass. What the TLS-50 is deficient in is mid-bass impact, speed, and upper-mid-bass (100-200Hz). The Watkins designed woofer of the Quantum 3 does perform exceptionally well in the lower registers. It very nearly matches the TLS-50 in extension, in my room. Further, because of the sealed cabinet design, the roll-off the low-end is gentler than it is with the TLS-50, giving the impression, on passages where mid's and high's are subdued, that it can play as low or even lower. Where the Quantum 3's really shine, by comparison, is in bass energy. The bass performance is subjectively flatter, and does not have the peculiar mid-bass dip that handicaps the TLS-50 in that range. Nor does the speaker seem to have the more typical bass hump at around 50-80Hz that many speakers use to imply energy or extension. For this, it has superior performance in rock and some high-energy orchestral works, and offers excellent performance elsewhere. Unlike the TLS-50, it does not rattle the walls with a 20Hz sine-wave input at only a watt; it requires 3 or 4 watts to do the same. One fear I had regarding the Quantum 3 was that I would have to use a different amp. I have grown to really like my Yaqin MC-10L II, and while I have some very good gear "in the stable", they don't possess the utter sonic bliss of the fully-tubed Yaqin. Infinity speakers, especially those with the Watkins licensed woofer, have a reputation of being very difficult to drive speakers and are colloquially known as "amp killers". It would seem, however, that the reputation is worse than the reality. While undoubtedly the Infinity is a difficult or unusual speakers to drive, the Yaqin seems to have no issues with it, provided that I keep the volumes sane, and use the appropriate 4-ohm tap. Indeed, the pairing is more synergistic, I might say, than the amp used to demo the speakers (a highly reputable amp). While certainly the combination will not be winning any SPL competitions, at volumes I normally listen at, and volumes I consider to be quite loud, the pairing never seemed to lack for power. Credit for this I give to the Quantum 3's moderate sensitivity. It seems, on the whole, to be at least 6dB more sensitive than the TLS-50, and I would not be surprised if it were even more sensitive. It is not, however, as sensitive as the JVC zero9, and it would easily take 50 watts of input to accomplish what something like a K-Horn would do with one. And so there are my initial impressions, handicapped as they are with substitute mid-bass couplers and more dirty pots than your favorite greasy-spoon restaurant. As usual, more listening will bring more comprehensive and fully formed opinions, and I look forward to getting another original mid-bass coupler so that I can more accurately judge the midrange performance.