i once had the same question,
at first i wanted to change from M8 into Mixwiz,
but i ended up getting me an A&H GS3,
i think the M8 sounds more clean than the GS3 i got,
my eqs do sound a bit better, have more routing options but the overal signal isn't as clean as in the M8
the M8 is one of the cleanest sounding little (analog) mixers i've used, a Mackie VLZ pro is **** compared to it,
all behringer stuff seems rediculous once you compared
Getting yourself a Mixwiz will give you a bit more options but i don't think you'll really upgrade the sound quality
My conclusion? i'd stick with it and get yourself an external Pre. Best thing is just a pre, without compression and eq,
used a few channel strips with no extremely great results,
my next buy will probably be a DMP3, simple and clean