I've been meaning to post back here to see if you'd received your amp yet. The Mazda tubes look like a pretty good deal, let me know what you think of them in your STAX preamp. Now that I've got about 30 hours on my tubes I'm planning to put the thing back on the bench to re-check the bias so I'll have a look at the tubes. I'm no tube expert so you'll have to tell me what to look for and where the "filament windings" are. I'll take a macro pic and put it up here. I'm enjoying the amp, no issues at all but that niggling doubt planted by the ARC guy is still knawing at me so I might try calling ARC back to see if I can chat to someone else who isn't so keen on getting me off the phone with a photocopy answer. Else I might still put the tubes on eBay for 50% of what I paid before they get too old and just get a set of =C= 6550's as originally used by ARC in this amp.