I have rolled several NOS tubes in the 12AX7 position in the Ref 330M monoblocks. Happily, the Ref 330M is sensitive to the tube in this position.
Some notes:
- Telefunken Smooth Plate - I seem to usually come back to this tube and it is what I am currently running. In this system it gives a nice musical balance - very powerful and defined bass with nice liquidity in the midrange and high frequencies. I like the musicality of the system a lot with this tube.
- Telefunken Ribbed Plate - the bass is tremendously strong and tight, even more so than with the Smooth Plates, and this tube seems a bit more extended on the top end. However, the high frequencies occasionally seem a little too strident for me on some recordings, but on John Rutter: Requiem & Five Anthems (Reference Recordings RR-57CD) it is difficult for me to imagine another tube that would out perform it.
- Amperex Long Plate Bugle Boy Type-I (D-getter) - the mid's and the highs are fantastic, but the bass is not as well defined as with either of the Telefunken tubes. The bass is there but it's kind of flabby.
- Amperex Bugle Boy Type-II - I've been told that these are a bit tighter in the bass than the Type-I. I have a pair on order and will update this entry after auditioning them.
