Rythmik subwoofer

I don't have Magnepans, but I have owned many subwoofers and I don't think you can go wrong with Rythmik, SVS, or also Power Sound Audio for well made "reasonably" priced subs. 

I've owned a couple of HSU subs and was less impressed with them.  I currently have two Rythmik F25 subs for movies and two SVS SB13 Ultra sealed subs for my two channel system. 

I also have some smaller "entry level" subs, a PSA S1500, a Rythmik F12, and SVS SB12-NSD.  These are all in smaller rooms and are more than adequate for that usage.

In terms of looks, I think SVS subs look the best.  In terms of sound quality, I really like Rythmik's servo technology, but SVS and PSA are no slouch in that department.  The best "bang for the buck" might be PSA.

Pick the one that works best for you in terms of size, budget, and looks and you should be happy with any of those three brands.

@jetter  I think the SB12-NSD is a really nice sub.  I'm listening to mine right now and for the small room it's in, it's perfect.  The room is maybe 12' X 15'.  It does a nice job of filling in the bass for my Canton monitors.  I have the gain set at about 9 o'clock and have it crossed over around 60 Hz.  It gets the job done and is not being pushed at all.

What you'll gain as much as anything by going up the SVS or Rythmik (or any other manufacturer's line) is more "headroom". 

My living room is pretty large and opens into my kitchen / dining area.  In that space, I have two SVS SB13 Ultras for my two channel system.  The SB12-NSDs would be working hard or not keeping up with my speakers in that space, while the SB13's are just coasting along.  I have the volume on the subs set at something like -7.  They fill in the lowest frequencies that my speakers can't adequately reproduce and provide that tactile feel of sound waves hitting you like you get at a live concert. 

For home theater, I add in two Rythmik F25 subs plus a HSU MBM-12 (mid bass module).  It's overkill, but none of the subs have to work very hard and it fills the room nicely when there are low frequency effects in the movie.

If you're happy with the SB12-NSDs and your room isn't too big for them and they're not straining, enjoy them.  Right tool for the job and all that.