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.
Duke
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