They won't affect your tone all that much. Especially not for high gain stuff. I have JJs in the preamp section of my 5150-II (identical to 6505+) and Svetlana Winged =C= in the power section. The amp tech I got to check over the amp and do the tube change/bias when I bought it said the Svetlanas were good and would do fine, but he preferred to spend a little bit more and get - something - I forgot... Anyway, the reason he said was that his favoured tubes lasted longer (in his experience). With that said, I've had no problems with these current tubes for well over 2 years now (and quite possibly a bit longer... oops).
I don't doubt that once you're down to tiny tweaks in your overall tone, the tubes do make a difference that you can probably detect, but for the vast majority of people (especially those with high gain amps), things like a string change will have MASSIVELY more impact on your tone ;)
So get some JJs for the Preamp, -something- for the power (lol, sorry, I'm happy with svetlana winged Cs) and don't look back until it's time for a swap and rebias.
Roo