[Stratrat]

I think the your problem is more with the pedal than the amp. Just about anything will sound muddy with a Boss Metal Zone in front of it. The Xotic BB Preamp that Allthumbs mentioned gets very good reviews, as does the MI Audio Crunch Box. Personally, I'm looking at the Barber Small Fry as my next pedal - it's pretty versatile and can go from mild to wild without sounding muddy and cruddy.

The Blues Jr. isn't a bad little amp at all. Sure, you can do better (if you're willing to pay the price), but you could also do a lot worse. IMO your money would be better spent trying a few pedals than looking to another amp....especially since you're only going to want/need those hard rock/metal sounds once in a while. If that was going to be your "main" sound, you'd probably be better off spending the bucks for a high-gain metal-type amp - but as you said, for just doing it once in a while you can always use the "R-Fier" setting on the Micro Cube.....or just find a better pedal to play with.