I'd say go with a Grado of some sort. It's about as different from an M50 as you care to be. I never quite had both at the same time, but the Grado came just after the M50, and it might have been the polar opposite. Mids were the focus on the SR225i I got, whereas on the M50 the midrange is a bit recessed. The M50 had thick, full bass; the Grado was light, quick, and punchy here. The M50 was not really colored but warmish and laid back in the mids. The Grado had a very characteristic coloration that, while not accurate at all, taken on its own merit was very pleasing. If you're looking for something that isn't more of the same, Grado is the way to go.