worth upgrading from REL Storm to REL Storm III?

Bdgregory, My Storm MkI also have dual connection (hi and low)actually 3 including balance input.

You may also want to take a look at the JL Audio 113 fathom sub.

My Storm MkI also have dual connection (hi and low)actually 3 including balance input.

true, but as far as I know, you can't use both high and low inputs at the same time. The crossover on the mk III allows you to drive the LFE using low level inputs off the sub connection from your pre-pro, and the music LF using hi level connection to your power amp, and do this simultaneously.

I may be mistaken here though.