Rel storm III or Stadium III.

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.

The Rel is old tech. Check the the Velodyne website and read about seemless intergration. It has a built in computerized room correction that is diplayed on screen. First do the sweeps from 15 to 200htz and see what your main speakers are doing in the room. Second,bump up the volume on the Velodyne sub. until it is close to where you want it. Third use the EQ to flatten out the room responce.
This is nothing like any other sub.