Rel storm III or Stadium III.

It this a home theater question? If you don't do much 2 channel listening, maybe you should take a look at the REL Q400E or Q401E.

The Stadium III is a decent step up from the Storm III. You get what you pay for..

I had an M&K MX-150 with VA Bach's and I too wanted a better sub for music. I just didn't have the crossover flexibility with the M&K. I picked up a Storm III and cannot say enough about how much of a difference it made. I always heard bass is supposed to be non-directional, or is it omni-directional, but with the M&K's high xover I could sense where it was coming from, not with the REL, I feel the kick and the bass sounds like it's coming from right in front of me, which it isn't it's in the same corner where the M&K was.
Can't comment on the differences between the two subs. I'm more than happy with the Storm. I guess it would depend on how much power you really need to match your room and main amp/speakers.
Call Sumiko since they import both brands. That is the same advice someone gave me a few months back. I took it and they answered all my questions even knowing I bought the sub second hand.
Since this is an HT forum, it rocks for movies too!

There is a REL product that is right in between these subs called the Stentor. Probably a very nice solution.

You may want to take a few minutes and check out the new Velodyne DD-15 or DD-18. The subs come with a room correction system that can totaly bring you to a new level.
This is the way all subs will be going.

http://www.velodyne.com/velodyne/products/product.aspx?ID=4&sid=906m399i

When you buy the sub,you get all the cables and mic. Takes about 30 min. to set up.

I have owned, and liked the one version older 18" Velodyne. It's a great sub for HT. It's a great sub period. But, when it comes to integration with your main two channel speakers for two channel music listening, go REL and don't bother to look back. I am now a happy Stentor owner (sold the Velo). The way the Stentor seemlessly integrates with my already full range speakers is amazing.

I run my big front speakers full range for HT, I only need that sub for the .1 material, and the REL provides seperate inputs and levels for both.

You have to understand your buget. Cycle per cycle the REL is a touch more expensive than the 18" Velo. The REL's higher price is commesurate with its higher performance, for sure. If the budget is a higher priority than music, go Velo. If music comes first, go REL.