Phase inverter 12at7 affects the amp globally on all modes and seems to make the amp tone more present and bigger yet also more vintage. It manages the signal before it hits the powerstage. But you can tell its done that to the core out of the box tone that still feel 'as was' in the attack and voicing. You definitely can notice the difference at any vol, but especially at volume. Channel gain/eq unaffected. Sounds more 3D sounding than stock.
The V2 on its own recenters the voicing of channels 1 and 2 but at a critical stage in the preamp chain which allows more gain, more dialling EQ and seems to revoice it closer to the old vintage sound (more prominently on Channel 2)...you can dial higher with bass, mids and treble at the pre stage without breakup/flub, for hairier gain settings, also the low gain sounds sound like they break up more pleasingly to my ear, also. (With more sweetness and "brannnnng!!!!")
Both chosen individually are great, but combined, its the warmest most authentic 'makes-it-sound-way-bigger-an-amp-than-it-is' sound ive heard. Luscious, 3d, vintage, diallable to exactly how you want it on the channel stages.
Presence can be dialled higher also without any unwanted frequencies being accentuated, allowing you open up the amp more in a more desirable/pleasing way.
Also key is a decent V1 tube. Like a Goldpin Tungsol 12Ax7 or Mesa SPax7. (The Tungsol feels more full rangey to me but both are excellent choices)
This will give an abundance of signal for the lower gain AT7 to deduct from and still leaving the amp soaring/walking tall.
The gain path like F1 racing hurtling at 200mph on a straight (The SPax7/Tungsol) then hurriedly slowing down for a sharp bend/chicane (AT7 effecting stage) before speeding up again after the bend keeping vital seconds shaved off the clock for the finish (12at7 then powerstage). Rather than driving round the whole track at a solid 90mph constantly. Weird analogy, but that how this feels to me.
Apologies for the late reply, didn't see this post 'til today... Hope this helps still.. The 35 definitely benefitted in the same way as the 25. (Did this in a friend's 35 for him, he was thrilled too)