FWIW I run my (modified, i.e. bass enhanced) AU-999 with flat tone controls but I do as a rule run neutral (bordering on bass strong) speakers (mostly Allison One or Allison Two speakers, some other brands but nothing bass-weak). I could see some folk might want to turn bass up a notch. For me I really love the AU-999 (modified) as well as the AU-888/666 which have a similar presentation, striking (IMO) a rather unique balance between the old boomy sound and the 'newer' hifi balanced sound with arguably more detail. With these amps, I sometimes have a feeling of lack of bass, but when the track/source really has bass, these amps provide it with fidelity, so I get what you are saying. It's the eternal struggle for me.... the desire for neutral clarity and fidelity and the slightly exaggerated woolly voicing of the older models that really gets the heart thumping (but does get in the way of ultimate fidelity and woofer control).
I ramble again, but for the reasons above I think I will have an 888/666/999 as well as a 777 or similar in my quiver as they sit either side of a clear line in history to me.... the singular biggest paradigm shift in hifi flavour
I get that and it is one way of doing it, but this is my reasoning for not going down that route of influencing the frequency response/curve of an amplifier.
As soon as you are boosting or influencing say in this instance, the bass, you start to also shift the phase, now this is something I really don't like doing in an amplifier. There is enough phase shift in speakers thank you very much, so if you are causing a phase shift in the amplifier, and then also mix that with what happens in the speakers, it makes a rather messy situation.
I am not being conceited, but I can hear that and I don't like it, even though there is more bass, to me there is now a factor in the sound I don't like. Same happens if you introduce an EQ filter into the mix, it shifts the phase, and to me then sounds odd. OK for one track that lacks a bit if bass, but as a standard system setting, its not the way I like to do it.
The designers have specified the feedback in conjunction with the gain of the amplifier and I'd just leave it as intended, unless there is an actual design shortfall like there is with the pre-amp. Even changing the cap in the negative feedback can affect the phase as well.
When I did a frequency vs Phase plot of the 999 I worked on the other day, the plot looked very good and personally I would not mess with it.
People never talk about phase, but in my experience its one of the most important things in sound reproduction.
At the end of the day, we like to try things and some folk may like this approach you are experimenting with, and thats a good thing for the community....and Kudo's to you for trying things out.
This is why I have an Active Bi-amped system as my main, with the MC452 on the mid/high and the Crown driving the 15" Bass, they aren't subs so to speak, they are a "low extension" to the 8xTi coaxial speakers.
This way I am using two amplifiers with perfect phase, and to increase the "fat", "warmth", "punch" whatever you like to call it, I just balance out the two amplifiers.
It's "set" as a system and I do not ever change the level of the Crown, I find I never need to.
Of course using a Linkwitz Reily 24db/oct Filter in the active crossover, there is phase shift at the crossover frequency, but it has minimal negative effect on the sound once the sources are aligned.