Sounds like an interesting concept... Any chance of a setting in the advanced page to have this available for all amps?
While I can't speak for what Cliff can and can't do or make available, I can speak for amplifier design.
In the Triptik, The initial bass cut happens after the first gain stage, but is incrementally brought back in through the next stages while also controlling and manipulating the high and the mid bands too. There are several filters at various points in the gain stages to manipulate these other bands.
One band is always relative to the other two. In other words if you want more bass, you don't necessarily have to increase that, you can attenuate the other two bands to create the same effect.
Keeping the bass out of earlier stages allows each stage to be set up for more gain, so you get more gain from less stages which keeps the S/N ratio down and the dynamic range up.
The feedback loop in the power amp is frequency dependent and allows more mid-highs to be passed back to the phase inverter on the inverted input out of phase with the normal input which flattens the response and lowers the power amp gain (closed loop gain) much more for just those two bands. The bass band is not attenuated anywhere near as much so is amplified at a level much closer to the open loop gain of the power amp.
So basically what I am saying is while it's clearly very possible for Cliff to emulate a resonance control on the power amp feedback loop, there is much more to it when constructing the frequency responses of each gain stage.
The first time I used a filtered feedback loop to get more bass and tighten lower end was back in the early-mid 80's when I was messing around with the old Parks, Marshalls and Sound Citys and modding them for myself and the lads back home in the North of England ! I first saw a filtered feedback loop in an ancient hi-fi amp my Dad gave to me to mess with, as well as some old Fender amps used frequency dependent feedback. This concept had genesis way before my time any most certainly decades before the new wave of Hotrod Marshall Modders were building amps. Messing with the frequency cut off points and the amount of feedback was something of a crap shoot at first for me, mainly because I didn't truly understand how critical all the stages were to get the results I was looking for back then and study resources were almost always restricted to the public library. Remember the days...no internet ?
Ironically the values I use now are very much the same as what I used back then