If you don't mind a slightly dark tone the EBS Valvedrive might be worth a try. It turns to mush if you crank the gain too much, but at the edge of breakup, and slightly past, it sounds great. It can get slightly dark/muddy if the rest of your setup is leaning towards a dark/warm tone, but it's a great pedal for adding a bit of warmth and tube-vibe to a sterile sounding amp. Of course it won't give you the "heft" of a real tube amp with output tubes and transformers, but unlike most "toob" pedals it actually has a nice tube driven breakup.