KT120 back to 6550

Seen a ton of threads praising the KT120 over the 6550 or KT88. I get the new tube's durability and design but given that your amp was originally designed around one of the older tubes has anybody gone back.

I recently purchased and ARC Ref 110 that came with a set of KT120s. The output tubes test and match well so they don't seem to be at the end of their life (same for the signal tubes). 

This was a change from a VT100 equipped with 6550s. The VT100 had good midrange but seemed a little lean otherwise. Percussion was fantastic and voices just right. Just deep bass a little thin. The REF 110 definitely fills in the deep bass but seem a little congested in the midrange. Both top and bottom are an improvement.

Both these amps were sandwiched between an LS25Mk1 and a pair of JBL 4430s. Could be a synergy thing, these speakers work very well with high power solid state (Adcom GFA 5802). I've been trying to work up a good tube amp for these but so far I'm not feeling the love.

So the question is could it just be this system would work better with 6550s instead of KT120s?

@saki70 : the 120s may seem to make the mids thin, but not if you have a preAmp that can compensate for that. 
E.g. I have the PrimaLuna Dialogue HP integrated tube amp, and swapped the stock 12AU7s for NOS Brimars and Mullard’s. The Mullards in particular lived up to their rep with midrange beauty with the KT88s.

In fact, it was too much midrange. So I think the Mullards with KT120s would then have better balance in all highs, mids, lows while retaining the glory of each. Just my experience. 

OP, I worked with Kevin and Jarrod at Upscale Audio on Matching tubes with my PRimaLuna HP Integrated (Kevin is part owner of PL) 

Through talking to them and reading the Upscale Audio website and watching Kevin’s short videos on many types of tubes , not only KT120s, KT150s, etc, but specifically tubes for ARC amps. 

Kevin has a video specifically for tubes for ARC owners. I don’t recall much of it because I don’t have arc gear, but he talked a lot about current, electricity matching with the electronics to not only get good sound but good tube longevity in ARC gear

-no affiliation-

I think PrimaLuna and Rogue have pushed Audio Research to come up with amps like the VT80. ARC power amps normally are seriously fussy about tubes which I'm sure was no end of frustration for Upscale. They've sold enough tubes over time to figure out what will work and what won't.

I'm sure the VT100 scares the crap out of them.

What Audio Research has been doing since like the mid 90s is direct coupling the driver tube (the last small tube) to the output tubes. So when you bias an ARC power amp you're biasing the driver tube which then biases the output tubes. If the driver tube goes weak or fails or is just way off from the tube you swapped out you lose bias and then it's all smoke and burned boards.

No tube fuses, we die like real men!

Meanwhile everybody else is going from EL34s to KT150s with a single credit card swipe, so you get the VT80 or a Rogue or PrimaLuna.

The plus side is the coupling capacitor moves back to between the last gain stage and the driver which I'm guessing helps with the sound they are after. But now there is more direct interplay between the driver tube and outputs which is why ARC power amps are so darn fussy about tubes.

If the driver tube goes weak or fails or is just way off from the tube you swapped out you lose bias and then it's all smoke and burned boards.
@monoogan This statement is false. If the driver tube dies, the power tubes are held in cutoff and are not damaged. If the driver tube is weak, you may have troubles getting enough bias current on the power tubes. So in this *particular* regard, the amp is fail safe.
lalitk, I would suggest checking with ARC first before rolling in the KT120's and particularly the KT150's. I would be surprised if they told you that your VT-80 was compatible with these tubes. Best to get ARC's blessing before you damage your amp.