If your Alpha is the 18 BIT model try the GE 5751 Black Plate or the Raytheon 5751 Black Plate. I have found those two sound different from one another, but will give a nice clear sound. With a 96/24 Alpha the Chinese 12AX7 is not bad. Some like the Mullard 12AX7 but I think they sound bland in the Alpha. The Brimar NOS 12AX7 is the tube to listen to, but prepare to shell out the cash.
RE: Tube swapping in Alpha DAC
Per the comments in a previous post: Swapped out the two Sovtek 12AX7WXT in my DAC with GE 5751. � vast improvements in top-end air: cymbals shimmered � good improvements in space: the sense of the recording venue improved � transient attack: let's just say that I was repeatedly startled by the content from very familiar recordings � tranparancy and detail: improved to such a degree that micro-dynamic shadings among guitars, piano, and vocals enabled a whole new level of immersion into the music; the decaying tones among some percussion were simpy mesmerizing � smooth: bye-bye tizzy asnd brittle top-end � width of soundstage diminished a very small amount; apparent depth increased by a significant, but not large, degree What a deal. The GEs cost less than the Sovteks ($8 vs. $12) and improve the quality of the music emerging from my system. The change was more obvious and significant than many wire swaps I have made.