AudioResearch used to be very cooperative providing upgrade kits to DIYers.
Nowday's I don't really know, but you should check if they'll cooperate to your endeavor.
You also need to know if it will resolve the bass issue(which I doubt), because bass deapth is limited by output transformer and there's not too much could be done there.
The great and definite solution is to add active subwoofer in the chain or couple. That will preserve great mid-range and trebble of tubes along with deapth of bass.
I'm not fan at all of bi-amplification, but in this case you can try combining your amps with McCormack DNA .5 that may mate with your VT100s without additional active crossover.
Nowday's I don't really know, but you should check if they'll cooperate to your endeavor.
You also need to know if it will resolve the bass issue(which I doubt), because bass deapth is limited by output transformer and there's not too much could be done there.
The great and definite solution is to add active subwoofer in the chain or couple. That will preserve great mid-range and trebble of tubes along with deapth of bass.
I'm not fan at all of bi-amplification, but in this case you can try combining your amps with McCormack DNA .5 that may mate with your VT100s without additional active crossover.