I have these monoblocks and was using the RCA cleartop 12au7s w/ Telefunken 12ax7. I recently switched to RCA 12bh7 to replace the cleartops and I've noticed an overall improvement. I'll probably use only the 12bh7 from now on. Many people prefer the 12bh7 preamp tube to any 12au7. Hope this helps. PS: I have some extra sets of 12bh7 if you have any problems locating some.
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