subwoofer question......what size sub ?

The trick about good integration of a sub has more to do with room integration than with the sub itself. Hence also, the proper size of the sub and driver are largely related to the size of the room. Pump too much deep bass into a small room and the result is pretty awful because of room modes. For that same reason I would always go for at least two smaller subs instead of one larger one, and always tamed by dsp room eq like the Antimode 8033. See here for a good first introduction: http://www.acousticfrontiers.com/20101029using-multiple-subwoofers-to-improve-bass-the-welti-devanti...
For music I would usually prefer a sealed sub. In a smallish room and on a limited budget perhaps two SVS SB1000s. In a larger room and on a bigger budget you may want to consider the Audiokinesis Swarm System: http://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/
Both combined with the Antimode, of course.

It sits between the amplifier (either at line level if you have that or with an attenuation cable at speaker level). It does not do low pass filtering or anything for you. You have to figure that out in the usual way. All it does is measure the room response of the subwoofer (an easy one time only automatic procedure with the provided microphone) and then applies an appropriate correction curve to the signal before it reaches the subwoofer. Since it removes bass peaks you may have to increase the sub’s level a bit, and that is all. See here for the manual: http://www.dspeaker.com/fileadmin/datasheets/dspeaker/antimode8033CinemaSIIen.pdf
I have found it very easy to use, and remarkably effective for relatively little money. See here for a review: http://www.theabsolutesound.com/articles/dspeaker-anti-mode-8033-dsp-subwoofer-equalizer-tas-204/
In my case, my main speakers are Quad 2805s, and they lacked a bit of deep bass. So I added a B&W PV1d, but found it hard to integrate the sub perfectly. So I remembered all those stories about fasts stats and slow subs, and feared I had made a mistake. Then I stumbled upon the Antimode by chance and decided to take a gamble, because the theory and the physics seemed right, and so did the room response measurements that they presented as examples. And indeed, the bass suddenly became much tighter, more tuneful, and ’faster’. What I learned is that fast and slow has nothing to do with the moving mass of the speaker but with the slowly lingering decay of the in room response that you can see in waterfall plots. Stats are fast because they are dipoles that excite fewer room modes, and subs are slow because they go low and thus excite many room modes. Deal with those, and the sub becomes almost as ’fast’ as the stats.
I have since learned that multiple subs are another good way to proceed, and the conclusion is, of course, that combining those two approaches is likely to give the best results.

You don’t need the more expensive Dualcore for two subs. The 8033 will happily equalize two or even four subs, but only in mono. That, however, many would argue is the best way to connect subs anyway. See here for connection examples of the various 8033 models:
http://www.dspeaker.com/en/technology/anti-mode-technology/anti-mode-connection-examples.shtml

OK. I agree that a dipole sub is a great idea (looked hard for one but could not find one at the time). However, why does a dipole sub benefit from stereo where an ordinary one does not? Deep bass is not directional, and the advantage of equalzing in mono is that the two subs even each other out more effectively. This really is an open question out of curiosity.
I am just occasionally toying with the idea of getting these dipole subs, but I am not in the US, and they also seem rather large compared to the B&W PV1d that my wife finds only just tolerable (and perhaps rightly so), given that we already have the large Quad electrostats.

That makes sense. I have been very pleased with the response from DSPeaker's engineers in Finland. I called them on the phone, and they took their time to explain what I could expect and what I needed (and what I did not need, even if it was more expensive).

True, after that it went in another direction. However, the first answer basically had it all: it is the room that you need to be concerned with. Since you ask, I have now found that elsewhere you give the room’s dimensions as 11x16x9 feet, and that makes the subsequent discussion all the more relevant. Room size is crucial information, and yours is very small: it gives a Schroeder frequency of about 200 Hz. Below that, you should expect room modes, i.e. hige peaks and the ocasional dips making for a boomy 'slow' bass. Therefore, if you still want to have deeper bass, you need to open the full box of tricks, i.e. perhaps bass traps, and certainly only small subs (and two or perhaps even four of them) plus dsp room eq. The dipole subs that were also mentioned in the subsequent discussion are another good idea, but probably they are physically to large in that room. But maybe you should just avoid subs in such a small room.
Have you ever measured current in-room response with REW and a Umik-1 calibrated microphone? I ask because below some 200 Hz your main speakers may also need some equalization in that room. For the ultimate solution, there is now the DSpeaker X4 pre amplifier/DAC room eq. But it would be throwing a lot of technology to a problem that is really quite simple: the room is too small for full range reproduction. That size is about the same as my study, and there my choice has been to limit myself to speakers that are great but lack the bottom end: the Harbeth P3ESR. If there is no bass, it cannot get out of control either.
And as I said, even without subs you may already have a problem with boomy bass and room modes. Measure response, and perhaps equalize the main speakers with REW into either the computer if that is your source, or a mini dsp.
If you do decide to go the subwoofer route, the Antimode 8033 is the simplest to use form of dsp room eq, and it works a treat. No need for user adjustability. But it will only work for the subs, and your mains may need something too (hence the expensive DSpeaker X4).
As for the subs themselves, I agree that sealed subs would be better than vented ones. Avoid anything other than the smallest ones.
Good luck. And be prepared for a smallish ideal listening spot. The laws of physics have not changed, even if dsp eq bends them a bit.

bdp24: absolutely. It is for the same reason that I like my Quad electrostats. Speed is indeed not a property of the speakers but of their interaction with the room. I wil look into the 8 inch version. Thanks