Check these out:
http://www.radioshack.com/product.as...%5Fid=23%2D037
A pair of those in a AA holder would be 7.2, and you could just use them to replace all 4 AAA's and still get the extra performance. Even better, being lithium, they weigh about the same as a standard AAA and less than rechargeables!
And to top it all off, you get more life out of them to boot (though they are not rechargeable)... 700mAh per AAA rechargeable (1000 for a standard), these crank out an impressive 2100mAh!
Also: Rechargeable NiMH 9V batteries actually put out 7.2V... (6 cells x 1.2) less life, but the same kick and rechargeable!