Re: 6505+combo tubes
I did change tubes in my 6505+ head back in the day.
For the pre-amp section I mixed both Electro Harmonix EH12ax7's with JJ ECC803S and JJ ECC83S. When playing purely metal, the Shuguang 12AX7B is a very good choice too (think serious gain).
As power-amp tubes I had the Shuguang 6L6GC-STR.
I must point out that I am not a fan of GT-tubes at all, all they do basicaly is relabel tubes, make a lot of marketing fuzz, and drive prices up imho, not saying they are selling bad tubes, but you can get the same tube cheaper, so I am not a fan of relabeling at all.