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?

Thanks
monoogan
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.
According to ARC, both the VT-80 and VT-80SE can run KT150s. Both amps ship with KT120s now. And all four tubes are fused.
Atmasphere

Bad assumption on my part. I'm guessing if the driver fails the output grids get pulled down by the negative supply cutting off the tubes. instead of just losing bias.

Sorry for the misinformation.
Tommuch, I see that the VT80 can apparently run with KT150’s. What is odd is that the newer VT80SE includes the KT150’s, and so I’m not sure what is the difference between the 80 and the 80 SE??