i would change all the tubes to NOS...i own this pre-amp too.
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I previously owned the 5.5 and I tried a few differant com-
binations. What I ended up with was: four premium Mullard
6201's in the 12AT7 positions, and two Amperex Bugle Boy
I did try Mullard CV 4004 in the 12AX7 position which were
very nice, but I preferred the Bugle Boys as they are more
more foreward and dynamic.
Once you try premium tubes in your 5.5 you will probobly
never go back to the stock tubes.
If you only want to replace one pair as in your suggestion,
it would be the 12AX7's. They are the front pair (left and
I agree with replacing the line-stage tubes (12AX7). I struggled with my system suspecting other components,speakers, etc. By serendipity, I came across a matched pair of 12ax7 NOS Mullard equivalents. Needless to say My search is OVER. MAGIC! I suspect that the tubes were not matched in the production of this fine pre-amp. Maybe they are now. But let me tell you I am enjoying the crap out of my system. Hope this helps.