Sub for Quad ELS-989

I have come into possession of a pair of Quad 989's and really love them but when I play music with any medium not to mention strong bass characteristics the protection circuitry kicks in and down goes the volume.
I thought maybe a sub might help this if I ran it through its crossover and filtered out the lower register but then I have a slow driver with very fast electrostatics. Is there a sub as well as a configuration that would help my situation?
Vandersteen 2wq, JL Audio Fathom (F112 I think depending on the room size), Rel B1, B2, or B3, once again depending on the room.
Macdad is on right trail.if you think your speakers are working properly but large scal orchestra or anything below 40Hrz or 30Hrz is setting them off best thing would to be to crossover the signal before it goes to Quads and a sub.You are doing HT so super depth a sound pressure level (SPL) is not the thing you's speed and therefore pitch definition.If you have a pre and amp getting a good crossover like a Bryston 10B Sub is not cheap but works great.Otherwise a Velodyne SVS-1 will do same AND allow for tailoring EQ to room.

Vendersteen is cheap and does not go to low but is a multi driver and faster.REL works in same way fast and speedy.Say a Storm or above (though entry Strata is ok).They have newer models (B series) but think older ones were better.There is a an off shott from REL MJ Acoustics that make similar subs.Some large single drivers will do job like Fathom but at a price and maybe without finesse unless you really pay for it.I onc spoke to Maggie folks and they weren't hot on subs but said don't use a large driver anything above 8" if you do go multi driver as above.Maybe most flexible sub is Martin Logan Depth.Has three 8" drivers and speed for electrostatics like theirs and yours.It's also got enough power to do home theater or work with wide variety of speakers.That and SVS-1 is set up I would go with iof I get back into monitors or ever went planar or electrostatics.Speed and pitch definition not "Boom In The Room" and means to high pass your Qauds is what you want.
Check out the Paradigm sub 25 or the new Sub One (a monster that is supposedly "fast" in that it uses 6 8-inch cones powered by almost 2000 watts). Either would be cheaper than the JBL and the room correction feature is awesome (although it doesn't run on a Mac).
The passive (speaker-level) highpasss filters found in plate amps are not designed with the impedance curve of an electrostat in mind. Also, they use cheap capacitors that in my opinion you do not want to put in series with your Quads. In other words, I'm suggesting you do something like either use an active crossover instead of a passive one, or build your own outboard passive highpass filter tailored to your speakers using high quality parts (easier said than done of course).

If you have a preamp with two sets of outputs, you could do this: Run one set of outputs to your sub(s), and run the other through a passive high-pass filter before it goes into the amplifier that drives the Quads. This filter would consist of a single high-quality, low-value capacitor. If you shoot me an e-mail with your amp's input impedance I'll calculate some suggested capacitor values for you.

Perceived speed in the bass region is primarily a function of the in-room frequency response and has nothing to do with the actual speed at which the driver moves. Because the ear has to hear at least one full cycle to even register the presesnce of low frequecy sound, and more than that to detect its pitch, by the time we hear bass the room's effects are in full cry. So we need to consider the sub+room as the system, rather than the sub in isolation.

Dipoles interact with rooms smoother in the bass region than monopoles do, at least according to an AES paper by James M. Kates. The technique I advocate for improved in-room bass smoothness is to use multiple small subs scattered asymmetrially (credit to Earl Geddes for teaching me this technique). Each will produce a unique peak-and-dip pattern at the listening position, and the sum of these dissimilar peak-and-dip patterns will be smoother than any one of them. In my opinion, this is an effective way to get monopole subs to approximate the in-room bass smoothness of dipoles.

Most rooms also boost the bass region due to boundary reinforcement (at least two authors claim 3 dB per octave below 100 Hz is typical), so in my opinion the subs' anechoic response should be approximately the inverse of this "room gain". I believe room gain is the primary reason a low-Q sealed box is often perceived as being more accurate than a vented box, as the low-Q sealed box will have a gentle rolloff across most of the bass region while the vented box is often "flat" down to a much lower frequency so the effect of room gain makes it bottom-heavy. These are generalizations and simplifications of course.

I have a TBI sub which is very fast and integrates seamlessly with my 989s. It fills out the low end without calling any attention to itself, perfect.
Check out the Martin Logan sub, fast and tight. Much better definition than the Rel.
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I have been expermenting and searching for an ideal(??) sub combo for my Quad 2905s and believe am almost there. As rightly pointed out there are 2 ways to go : using an external crossover like a Bryston 10 Sub or a Marchand to block out all signals below 40hz or so. This has the reported advantage of letting the Quads play a lot louder without the protection circuit kicking in.
I, however, chose to go without an external crossover. Have experimented with a variety of RELs. The advantage of a REL, IMHO, is that its fed from the speaker terminals and can be tuned to kick in just below the lowest frequency of the 2905s ie 27hz. The RELs kick in around 30hz, hence there is no smearing or overlap between the subs and the main Quads.
Currently am using a REL 505(500watts) and a 305(300watts) in a room : 24x14x10. I plan to match the subs and eventually acquire a 2nd 505. I wonder if this will make a huge difference.
The results in a largely vinyl system are pretty impressive and to my surprise the Quads are playing louder today than ever before. I don't understand the explanation for this eg the numeric display on my Conrad Johnson preamp has a max reading of 99 and I can drive the Quads to 96 or 97 with vinyl as the source. This never happened before the subs arrived.
IMHO the RELs provide a sweet and tuneful bass without being over powering. The holographic midrange of the Quads gels perfectly with the RELs in the context of musical genres I value.
The Quad JL Audio route is also highly recommended but thats something I was unable to pursue.
All the best.
I use the same electronics as Bob's friend (Velodyne SMS and NHT x-2) with Rythmik subs. This combo works well with Maggies - not Quads, but cut from similar cloth (if you'll pardon the pun). I suspect that there are many fine subwoofers out there that will work well, but I attribute most of the credit for my successful set-up to the aforementioned electronics.

Good Luck