Not to sound contrarian, I think the sub IS the best bang for the buck (short of repositioning the speakers, if that does the trick) to solve your problem.
There are several great subs in the budget price range by Hsu, PSB, SVS, etc.
Also, I've never found EL34-based amps to ever have extreme control over bass. It's not their strength. But the midrange is usually very good. Work to the amp's strength, and let the sub take care of its weakness.
Just my 2 cents.
Michael