Its simple, the PL3 and the PL amps are very synergistic, and combined they challenge a LOT of stuff out there, even some fairly expensive seperates or integrateds.
I use the PL3/5 combo in preference to other stuff on hand, and I'm considering moving on to the 7's (I've never heard the PL4's or 6's as I'm not all that fond of EL34's). For all the accepted beliefs of their sound, I always find them a little peaky in the upper mids/low highs. Not as linear as say a 6l6GC, or if you are up to doing a little re-baising (or checking your current settings) 6550's. But FWIW I do like my PL1 with SED 6l6GC's. Depending on what you think of your PL6's with EL34's you might try them. And FWIW, you could even use KT88's if you want something a bit more EL34 like, with balls, or 6550's with a bit more of the 6L6gc linearity with bass balls. Again you might have to reset the auto bias circuit.
Apart from each other I think the PL3 is a tad dark, at least it was in other SS and tubed amps I matched it to, and seems less than optimum resolution wise, but together this apparent 'darkness' or lack of resolution, is picked up by the PL5. In a 'rave review', I think of the 7's, the reviewer observed in an almost off handed way, that the amps sounded better with the 3.
The only difference between the 6 and the 7 is the speed added by mod's to the 6 and, I assume, an adjustment to the bias curcuit to accommodate the larger power tubes. (Contrary to PL's ad's, while all tubes mentioned will work in any of their amp, they do NOT claim that all of them will operate optimally).
Excuse the stream of consciousness (or un). It all just fancy guess work based on a few facts and limited personal experience. :-)
You could though, if you're so inclined, tell us what you find about the sound of your present combo that you want to improve. Then we might give you some specific, and hopefully meaningful, recommendations.