I don't know what they cost, but I imagine you could get a single Kappa 15lf for the price of two of the betas. Run it five cubes (140 litres) tuned to 40 Hz with a pair of 4" ports. You'll get great output, and this thing goes damn *low* all things considering... To me it sounds flat to 30 Hz almost. It's one of the best low frequency drivers I have found from eminence that isn't too pricey and will run in a pretty small box.

You'll get more output than the betas (same sensitivity, but no power compression) with flat response, good sound and surprisingly deep bass.

Adam