You have several options:
1) MTX is not a bad sub, 10" will deliver tight accurate bass, I'm not sure which model you have, but you are probably underpowering it. A bigger amp will probably help. If you have the 6000 or 8000, you should be able to get some decent bass out of it.
2) If your looking for SQ, with good SPL, you might want to check into a bigger sub or multiple subs. Alpine, Infinity, and JL Audio are some of the more affordable subs, although the MTX 6000, 8000 series are capable of decent SQ and SPL. So adding another MTX 10" would help out somewhat, but you run into the bigger amp problem again.
3) A 12" sub will give you deeper bass, than will a 10", including more SPL, but 2-10's most likely will produce more SPL than will one 12"
Your amp isn't a bad amp, but it's not a quality amp either. You will definitely need a new amp to achieve what you want.
If you are interested in new equipment, how much do you want to spend, we can give you suggestions and places where you can find good deals.