I have two amp heads with 12AX7's (probably Chinese or whichever is cheapest). The have worked reliably for countless hours at high volume, night after night. Ryder truck after Ryder truck, for over a year. But I want to replace them before I'm onstage and go through a melt down. My central question is; If you were going to replace these tubes, what would you use??? (I like a fat, Aguilar-type tone if that helps) I've heard/read - get rid of your 12AX7's and get 5751's - they are direct 12AX7 replacements that blow 12AX7's out of the box. I've also heard that the "tube in the preamp" deal is just bull from the 90's. I don't even know if a replacement tube would make a perceptible tonal difference. Then there's the whole "Russia"/"China"/"Czech"/"Pago Pago" factory thing. I'd very much appreciate hearing from any of you tone surgeons about what you might consider in the same situation! Thanks for any input!