Having my Alpha (non 24/96 version) for agood two years now, being bought used from Agon, and is a '93 model. I've never had it eat tubes yet, but will admit that a pair of unbranded, supposed Shuguang 12AX7 C-9 production Tubes went "fuzzy" in a hurry, and it didn't appear to be any fault of the Alpha, but rather cheap, junky tubes. BTW, they didn't fail, just sounded as i describe, raspy, and fuzzy, so out they came.
About a good year ago, I ordered a number of pairs from Kevin at Upscale Audio. NOS '60's GE 5751, NOS '50's RCA TM Black Plate 5751, and a pair of the Groove Tubes 12AX7M Mullard re-issue.
Then lastly, did buy about 6 12AX7 Sovtek Mullard Re-issues, and tried these as well.
In my experience, the '50's NOS RCA 5751 had the best sound of all, but surprisingly, the Groove Tube 12AX7M was very good as well. The New Sensor/Sovtek Mullard re-issue is a good tube, and bang for the buck, but I do like the GT Mullard Re-issue ever so slightly better, it is not quite as dark as the Sovtek. Both though are good tubes that won't break the bank.
The 60's NOS GE 5751 I got from Kevin were good as well, but just a little bit more forward in the mids, like the singer stepped up a bit closer to the Mic.
Virtually all who have owned the Alpha, and have gone through the trouble, and expense to do considerable tube rolling with this unit, will tell you the NOS Sylvania Triple Mica Black Plate 5751 is the absolute holy grail for the Alpha, as well as other applications, but there are downsides, being that you will pay horribly through the nose for a pair of the real deal, and it has been said by those who have experience with these Tubes, that they are a tube that does not possess a long life.
My best, bang for the dollar suggestion-advice, is try a pair of the matched, platinum graded GT 12AX7M from Kevin from Upscale Audio.
When I got the Alpha, the original owner had a pair of JAN/Phillips 5751 in place, and while they might've sounded fine in his system, they were so unbelieveably harsh, and thin-glassy in my system, it literally sounded like fingernails on a chaulkboard. No warmth, and poor Bass Response as well.
I also one a Sigma II, and in this DAC, the Sovtek 12AX7LPS has worked very well, and has been in there for quite many years with no issues. You may find in the Alpha, the LPS Tube may sound a bit thin-cold. Mark