When I was doing the same to my TL12+ amps I fiddled with the gain and felt it was better after, but reverted before sale for authenticity.
What made the biggest difference on the 'scope was tweaking the feedback components. Tidied up the 10kHz square wave quite nicely! No idea about sonic change though 🤣
The sensible way to attack it is to do both, as I'm sure you already know. Changing the NFB is the simplest way to drop the gain, but the RC network needs to be bang on to keep phase coherence. I used 1% silver mica caps and closely matched resistors but kept the original values. Testing shows a nice square wave at 1kHz and 10kHz, so I'm happy with that. Changing the feedback resistors needs the capacitor values to be changed as well, of course. The easy way to do it is take a stab at the equation, then put decent caps in and a pair of trimpots to get it spot on.
Once I've sorted the third one, that'll leave two that are cosmetically dire. I won't feel so precious about keeping those stock, so I have a couple of thoughts. The really mad one is to parallel them to run as monoblocks. That won't double the power, but it will give more current and more output before distortion kicks in (if I've worked it out right). It also means setting each pair of OPTs to 16 ohms to match an 8 ohm load. That has a major benefit, because they're wound ideally for 16 ohms so the whole ouptput stage should be a lot happier.
Or I'll just respray both, rebuild them as standard and start moving some stuff on. I need the space. As much as anything, to finish all the rest. Anyway, that's just the Stereo 20 collection. I haven't even started unboxing any of the rest yet