subwoofers and panels don't mix

Eldartford is correct in saying you need large diameter short excursion cones.Most mainstream subs using long excursion drivers with heavy rubber surrounds are going to sound slow and smeared.They are just wrong.
I can recommend the RCF L15 /554k 15 inch woofer[superb quality Italian pro speakers] as a great driver for using with panel speakers.Put these in a 100 litre box and port to RCF recommendations and drive with any half decent subwoofer amp and you will get that extra octave of bass[33-66 hz] that many panels lack.And it will integrate superbly and be fast sounding.Also this drivers high sensitivity means something like a 100 watt sub amp is more than adequate.

A couple more possibilities:

1) Consider stereo subs. IMO, even at low X-over points, stereo subs create more realism. See the 6moons article:
http://www.6moons.com/audioreviews/vbt/magellan82.html
Also, consider a pair of VBT/TBI subs as an option. They're small and inobtrusive and can be placed anywhere and have short excursion drivers and I've found them to integrate well with several speakers.

2) If size doesn't matter, consider a pair of dipole sube to match your dipole speakers, and place them in the plane of your Maggies.
I first realized the value in dipoles when I had a square room and big Proacs. Couldn't get the bass right with any placement. Switched to Genesis V with integrated dipole subs and the bass improved dramatically.
I'm using DIY Linkwitz Phoenix dopole subs and can't say enough good things about them.

3) Consider Tact preamp room-correction. Maybe more than you want to spend, but I've not heard anything that integrates subs with the mains as well as this. I was astonished the first time I heard it. Nice and fast digital crossovers allow you to position the subs where the bass sounds best and corrects the bumps and valleys of the bass frequencies at your listening position.

When you see the aberrant bass measurements in your room with the Tact you'll understand why so many folks abandon subs---their loss. Low frequencies add spatial cues that add so much to realism, but they have to be tamed so they don't sound out of control (and muddy up the mids). The Tact does this amazingly well.
Good luck.

Martin Logan Depth sub and 3.6 Maggies sound fantastic in my room....crossed over at 35Hz.

I find it very hard to belive the DD series did not work well. They also have very good tech support.

Tim

I think that most people buy subwoofers largely by the numbers, and the numbers aren't kind to transient response optimizing designs.

Consider two subwoofers, identical in size and price. The first one is -3 dB anechoic at 40 Hz, and second one is -3 dB anechoic at 20 Hz. Which one do you think most people are going to buy?

What the numbers don't tell you is that the 40 Hz subwoofer has much better transient response than the 20 Hz one. Good transient response is expensive in terms of box size and bass extension.

I'm involved in the development of a subwoofer designed specifically to work well with Maggies, and if the project is a success it will probably be marketed through Audiogon. The design will be optimized for transient response, rather than for loudest deepest possible bass in smallest possible box.

I do not think that the surround itself is a dominant factor in the sound of a subwoofer. Rather, those subwoofers you see with the donut-like surrounds are typically optimized for loudest deepest possible bass in smallest possible box, and transient response is the casualty. The prosound 15" woofer mentioned by Jtgofish has parameters more conducive to good transient response, assuming adequate box size.

Duke