Alright, I'll play.
Since the OP asked about tweaks and mods, at the risk of being excoriated, I'll start with the only real "mod" I've tried: Tube rolling on my CJ amps.
I have a box of different power and input tubes, new and NOS, and it was a blast getting in to tube rolling.
My CJs originally came with GE 6550s and once those weren't available anymore I would just take CJ's advice and used the new Tungsol 6550s. (The power tubes tended to last at least 6 years or so). Though when my previous Tungsol died, I happened upon a killer deal for two matched quads of NOS Svetlana winged C 6550s. A lot of CJ (and other tube amp) owners were all about the newer, larger Tungsol KT120s and I tried them and....I don't dare go in to my impressions...but I'll say they ain't leavin my amps and here's a photo. (I also like, aesthetically, that they are even larger tubes than the 6550s...just for the more tuby look).
This is one of my Premier 12 monoblocks with the larger KT120 power tubes:
Secondly, and hopefully less controversial...
....
The other way I tweak tends to be playing with acoustics - positioning of my stereo speakers, adjusting sidewall curtains for reflectivity, and a while back I started placing a curved diffusor behind and directly between my loudspeakers. This has friggin' revolutionized my sound, IMO. It seems to add more focus, liveliness and density to the imaging and sound quality.
Since my stereo speakers are pulled out in to the room closer to the listening sofa, and my home theater screen and L/C/R speakers are behind the stereo speakers surrounding the screen, that leaves my large center channel behind and between my stereo speakers. So I either place the curved diffusor lying on top of the center channel, or on the floor aimed towards me - different sonic effects.
Here's the brown curved diffusor laying on top of my (velvet covered) center channel:
View attachment 336443
View from sofa, between my L/R speakers:
View attachment 336444
Another way of tweaking the sound in the room, playing with increasing or decreasing reflective surfaces, is with the projection screen shown above.
The screen itself is far lager than in the photo, almost the entire size of the back wall. However, it is surrounded by a 4-way automated masking system of black
velvet. Using my remote I can adjust the size and shape of the screen, from all the way covered up by the black velvet, to all the way open and anything in
between. So I can dial in just the right amount of added reflectivity to get the sonic qualities I'm looking for, in terms of liveliness, imaging, timbre/tonality...