Hello Maxdimario,
even the V72 circuit looks a kind of simple, do not get fooled. It is a perfect harmony of all components, so you can not just change or add a part to get some more gain without throwing off the balance.
Please take a look at V72/77/78, all pres build around the same concept, with different max. gain and most of all sound....
The IRT V72 and 78 are identical chassis, same x-former with different hook up, so that means different impedance, response and sounds vary quite a lot...
The V72 input coils are hooked up in series, the output in parallel, +0/-0.5 dB 20/20 (the S version). AC/DC negative feedback, plus a positive feedback to tame the resonance, very stable at 16 to 10k Ohm load...
The so called EMI version the V72S, input is hooked up the same than the IRT but the coils are wound different and mechanical in phase, instead the out of phase of the IRT version. The circuit is nearly identical only two resistors are different and with a higher value plate inductor.
The V78 input coils are in parallel, the output in series, you gain 18 dB by trading off the stable frequency response to a more home stereo spec of +/- 3dB 20/20. Slight modified feedback control (adjustable), no positive feedback, very unstable (you mentioned sluggish) at different loads, (needs about 1.5k output load)
The V77 has a different input x-former (similar winding style, higher ratio and cross coupled) but again hooked up in series, the output in parallel, a slightly different feedback, but stable at 50 to 5K loads....
95% of all V72 units are IRT speced and have a frequency limiting x-former, both high and low shuts off or better roll off quite fast... due to the way the coils are hooked up.
The first step is to change the x-former to the "S" spec version in order to play with the gain...
Most (home) made gain mods, are changing the DC controlled feedback, or just cutting out the DC (inserting a cap) while trying to control the feedback with an potentiometer. This will pass signal, but has nothing to do with the very high quality sound the V units are known for. Mostly you end up with a high end boost of 6 to 8 dB an octave, reducing the S/N ratio by 20 to 30dB.
Answering your question is quite tricky, and even I got help from one of the members of the historic V72/75/76 design team, it was quite troubling coming up with something that works, does not change the sound and is easy to implement.
In order to keep the unit stable the DC feedback (negative and positive) needs to be intact, you can decouple the AC via a cap, and with two buffer resistor control the AC feedback.
Still this only gives you a max. of 50dB, but at the same time you have to compensate the frequency rise (+8dB an octave) if you go over 40dB...
Changing the x-former strapping is out of question otherwise you change the very stable frequency platform plus the in/output impedance.
Going further into detail is proprietary info, but putting it in an easy way, in order to change the ratio of the output x-former to 600 Ohm(you gain the missing 10 dB) you need to change the inner impedance of the 2nd stage tube....
Best regards,