If you're not going to change out your whole rig I'd just suck it up and buy an 8x10 - or a pair of 4x10s. Probably the only cheap way you're going to get a bunch more volume out of that setup is to throw a monster cabinet at it.
If you're willing to drop some decent change I'd suggest a 2x15 with Kappalite 3015's, vertical, possibly with a mid driver. Pretty much the most sensitive, light, affordable combo I can think of for a low wattage head. It'll come to 4 ohms, ~104db paper sensitivity, and ~60lbs, if you get it from LDS.