You could also try a powered sub (even better, get two and run them in stereo, just like the monitors). That might take some pressure off the speakers and amp.

If you want to step up to floor standers, get ready to lay down some dough. If you spend the same that you did on the monitors, you'll have more bass and less of everything else. I personally like floorstanders better - I don't like looking at stands - and I'd rather put the money towards a bigger box. Some nice suggestions above.

Insead of running them into the ground, I would sell the monitors on eBay, atleast get a little money for them, and start enjoying your new and improved system earlier than later.