Twangntweed I also have a CVR and am currently looking to change my tubes also. But first let me say that this amp stock is a tone monster. The only knock on the amp is that most are noisy(loud hiss), and I beat that by simply changing the GT preamp tubes. Really the power tubes and stock P10Rs will get the job done tone wise. But I am changing my power tubes just for experimental purposes, just to see the effect different power tubes might have on this amp( never changed power tubes in any amp). So this will be a first. I did take the amp to a tech to be biased because the rating of the tubes that I bought were lower than the stock GTs (rating 8-10 for the GT red labeled tubes). He said that the amp has a fixed resistor that has to be changed to bias the amp. Now the company that I bought the tubes from claimed that if they could match those tube ratings (8-10) that chances are the amp would not have to be biased. But I didn't know that when I ordered the tubes. Job not done yet. Keep in touch and I'll let you know how it went. My tech on holiday. Will be back 28 Dec.