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:QBB:
+1
IMHO you'll have the same problem with a 6505+ as you do with the MkIII--either a great rhythm or great lead tone. Usually what separates the two in my limited experience is the mids and treble. With a good EQ in the loop, you can dial in or out the frequencies you need. Kinda like having a third channel.
You could also try a BB Preamp for your leads; it has a Bass and Treble control, a SHITLOAD of boost and gain, and is very warm and smooth if you set it that way. You could dial in a cutting thrashy rhythm tone, kick in the BB with the treble rolled off, bass up a tad, and the volume cranked and you'd have instant gratuitous sustain

The best part is that it's so transparent, it sounds like an extra tube gain stage in your amp.
Of course, I just got mine, but so far I'm amazed at all the different tones it can get... almost all of them good