My personal experience, having owned a 66-J since early 1968 (mine appears to be '63 based on EIA codes)...
This amp is NOT a Fender tweed Deluxe. If one sounds like that, something is wrong with it! Think Fender Princeton, non-reverb; any distortion you hear is coming from your ears...or maybe the speaker? This is a loud and clean amp which belies its pair of 6V6 tubes
that should only make about 14 watts RMS. This is a great amp, but not what one would expect from an early 60's tube amp...Steve and I have discussed that maybe the "J" ('cause there was a "66" earlier) meant "Jazz" and so was made to be clean for those guitarists?
I think I would recommend the later "Thunder I" or "Thunder I Reverb" which though physically bigger and perhaps a competitor to the Princeton Reverb, it really wasn't, probably 8-10 watts RMS from its pair of miniature power tubes (6BM8's? Help me here Steve!)
Is there a 66 schematic somewhere to compare? I took a look at the 66-J (actually 99 J) schematic on the Prowess site. There doesn't seem to be any 'thrown away' gain there. The instrument volume control setup is kind of wacky, like the 5E3 Deluxe, but the mike channel is even stranger, and I'm trying to make sense of how it interacts with the tone stack.