Well first of all, phooey on me! It is (some of) the Fisher tuners that include the test jack, not the receivers. See? That's what I get for responding from a remote location and relying on memory only as to which units included the jack! I apologize for the confusion. So, now that I'm back at home, then:
Second, I'd like to confirm exactly which type of unit you are working on. The later unit does use three 6AU6s, but those models ALSO include a 6BA6 tube in the IF strip as well -- except that you didn't mention that tube. On the other hand, the earlier units do not include the 6BA6 stage, and use only two 6AU6s and one 6HS6 in the limiter stage. If this is the unit you are working on, and you are using a 6AU6 in place of the 6HS6 (in essence, three 6AU6s again), then performance will be compromised. The 6HS6 has basically twice the Gm and amplification factor as does that of a 6AU6, and for those models without the extra IF stage, using the proper tube is a must.
OK. So now I'm back to my original questions. Does the unit quiet properly so that there is no static or rushing sound behind the audio signal? In other words, is the signal otherwise normal other than having to turn the volume up much higher than normal to produce otherwise normal sound? And is this the case when FM Mono is selected?
If so, then the problem almost certainly will be located around the final IF strip (ratio detector) transformer (Z4 or Z5 depending on which model you are working on). It is in these transformers where the audio component is actually recovered from the FM signal.
In the case of a normally captured FM signal, the audio output level is not dependent on the strength of signal received, but on the frequency deviation as created at the transmitter. Since we can assume that the station is not the problem, then the problem must be associated with the ratio detector transformer that recovers the audio.
Fisher included the detector diodes within these transformers, so it may be than disassembly is required to test those.
Let us know what you find!
Dave