due to the way the 5150 is configured, anything you do to the clean channel with affect the dirty channel's tone too.
I've just swapped my single channel 6505 with my mate's 5150 mkII and the mkII is a MUCH better amp. the clean channel is much cleaner AND you have a footswitchable crunch channel. lastly, the dirty channel is tighter than the 6505 and has more note defination.
if you want a clean channel I'd say to sell your mkI head and get a mkII instead.
:twisted: