I think my CAL Alpha DAC is broken...

I've owned the 24/96 version for at least a dozen years. Lately, it has become very temperamental about what tubes it will accept. The left channel blows out anything other than a Sovtek 12AX7WB! I've had the following new-out-of-the-box tubes fail within the past year: Sovtek 12AX7LPS (3 of them!), TungSol ECC803 and Sovtek 12AX7LP. Typically, they last only a few hours.

I'm eventually going to take it for repair, but I'd appreciate any thoughts on:

a. why it might be doing this now?

b. why the Sovtek WB seems more robust?

Thanks.