Audio Research VT100 upgrade to VT100 MKII

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.

The best update was to the VT100 mkIII as it did away with the innerstage DC balance adjustments which can be a real pain. Most people with the MKI and MKII just replace the front end tubes and output tubes without making those adjustments and it's really important as it sets operating points for the front end of the amp and effects distortion products.

Cygnus ... just a cost benfit observation. I don't know if ARC will still upgrade VT 100 Mk I amps to the Mk III version. Even if they do, here's my challenge point.

I saw a VT 100 Mk III listed on A'gon for $2700+ or there abouts. You might be able to do better by selling your amp as is and then springing for the Mk III, just being out of pocket for the difference.

Another idea to consider is the VS-115. I used to own the VS-115 and I thought it was a fine amp. Built like a tank, but had real finess. Some say it sounded about 85+% as good as the Ref 110. I've seen the VS-115 listed on A'gon for $4000+ or so.

Incidentally, the VT 100 Mk III's tube and input stage topology is similar to that used in the VS-115. Namely, 6550 power tubes and 6H30 drivers. ARC swapped out the 6550s for KT-120s in the VS-115. I don't know if the VT 100 Mk III was set up for KT-120s -- could be too much current draw for the power tranny.

Thank you for all your replies. I don't know how big the difference between VT100 and VT100 MKIII, or VT100 vs. VS115? There are not a lot of positive feedback on VT100, but I heard good things for VT100 MKII/MKIII. I feel comfortable on biasing VT100. If there is not a big difference, i'd rather keep VT100. Do you think NOS driver tube on VT100 can significant improve the SQ on VT100?

I would ask Kal for his advice. Kal is ARC's customer service rep and is very helpful. I am sure Kal can guide you to make a good decision that will make a lot of sense froma cost-benefit perspective. I just spoke to him today about a different ARC component. He was very helpful.

Big upgrade jump on it now!!