subwoofers and panels don't mix

I have heard Nola Thunderbolts sounding very good with Maggies. The TBs are very fast and transparent - musical is a term that gets tossed around without much meaning but it does apply in this case.

When I sold my last pair of Maggies ten years ago, alas, the TB was not available.

Hmmmmm, I have been thinking about adding a sub to my electrostats. I had not thought about the above problems. Now I probably won't try as i can do better without than a poorly mated 'with'.

There's a basic rule of thumb that usually works very well: Cross over the sub one octave above the -3 db of the main's low end, e.g., if you're mains are -3 db at 40 Hz, set the cross over at 80 Hz. This counters the conventional wisdom of crossing over as low as possible. I had the same problems when I persisted in crossing over low. You will need an SPL meter, test CD, and some time to get both the sub's level and phase set properly for the flattest response in your room. I was even able to tame my room's 60 hz mode. I don't buy that subs can't be integrated well with stats or panels. It takes some time and experimentation. Don't assume just because an audio store or someone you know has plowed megabucks in their system that they know how to get a sub integrated properly.

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.