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?

I believe that if an amp can work well with 6550's and can also accommodate KT120's...then the KT120's would be the superior tube in that amp. However, the big question is whether the amp can actually support KT120's without damage...as most that were designed for 6550's cannot. I went from an amp with 6550's to a mono block with KT150's...very big difference. 
Monoogan ;

I had the same experience when I tried KT 120's in my Prima Luna . 
Pretty good top and bottom frequency output but the all important midrange left me wanting . 

Good luck

Thanks every one. This is actually pretty helpful.

So for Saki and Twoleft it sounds like the tradeoff is maybe midrange definition versus extension at both ends? I think that's what I'm hearing.

As a side note towards the end of Ref 110 production Audio Research started shipping the amps with KT120s instead of the 6550s. As far as I know there was no other change to circuit or transformers. I assume they liked the way they sounded in the amp.

Of course they may have just been running out of good 6550s.
I recently compared Genalex KT-88 and Tung Sol KT-120 in my Emotive Audio Vita amps.  I have been using a set of KT-120s for the last 5 years but recently one of them was making some suspicious noises, so I swapped in my well-broken-in set of KT-88s while I waited for a new set of KT-120s to arrive.  Bottom line: in my amps the 88s lack the bass weight, power and authority of the 120s.  They don’t sound bad but the 120s sound a lot better. 
@salectric that was pretty much my experience too.  In a tube integrated, you can take care of the sweetness factor with the pre-amp tube(s).