Anyone can spend $100 and get a very good bottle of Cabernet, the art is finding one even better for $20… Why are the marantz units so highly rated? Is it because millions were made in the 1970’s?
I have limited experience with Marantz, hearing just the 2216B, 2230, and 2245 extensively through OLAs and a few lesser speakers, but to my ears, the Marantz sound is really something special. My only problem is that my Marantz receivers are a little underpowered compared to other brands, especially for using with my OLAs. Of the mid-powered Pioneer, Sansui, Philips, and Realistic receivers I've tried (along with a few other brands), the Marantz sound beats them all--to my ears.
The Sansui 4000 had the edge over my Marantz 2245 because it drove the OLAs better, but I'm sure a higher-powered Marantz would beat the Sansui sound (not that the Sansui sound isn't something special, too! It is.).
The only satisfying mid-powered receiver that I've found that can hold a candle to the Marantz sound is my Luxman R-1030. That 30 watt Luxman drives the OLAs better than my 2245 and it has a really sweet sound, much along the lines of the Marantz but with a better overall balance and consistency. Still, I've found nothing so far within my budget that can match the Marantz-OLA pairing for vocals and acoustic guitars, which are really important to me. The Luxman is dang close, though. I have a hunch that an 85-125 watt Marantz would beat the 75-120 watt Luxman in this line, but it would be a good fight. So I just think the Marantz sound is really, really good--for me and a lot of others. The expense is what limits me.
I guess to get back to the original question for this post, I'd say that there might not be any underrated receivers that can match the brands we already think or know have the best sound. Luxman is known for great sound, so I don't think it's underrated as much as lesser known, like DSwankey alluded to above. So it's my opinion that we'll only find underrated receivers that can be found for less money than the best receivers, but they just won't quite match the best vintage receivers out there. (Sorry, this turned into a long post.)