Subwoofer advice please

I am familiar with both the REL and SVS subs suggested.  You can't really lose hear, but for music in your system I would suggest the REL over the SVS because it was designed to operate effectively with your high level out, and is tuned for both music and low frequency effects, while the SVS is designed to fully take advantage of low level signal in a home theater or in a combined Home Theater/music set up.  Both sound very good, but I think the REL is qualitatively a little better.  Another option is the B&W PV1D, similar in price to the REL.  I am using one of these in my main system in a room that is about 18'x12'x8' and it sounds great.  It is wired for high level input off the speaker terminals for analog and 2 channel music listening and with low level RCA input for 5.1 low frequency effects.  Pressurizes the room well.

I have owned Quad electrostats for the last 37 years, and most recently the 2805. I think they are the most natural and vanishingly neutral speakers that there are. Some years ago I decided that in our new and rather larger living room we might benefit from a subwoofer (the 2905 was not an option because it would block the view through a large panorama window). So I bought a B&W PV1d as the sub that was claimed by many to integrate best with stats. Yet I was disappointed: the bass was woolly and ill defined, and quite different from the crystal clear sound from the stats. And indeed that is precisely what many say about integration of subs and stats.
So I started reading and doing some research into the science of reproducing lower frequencies, and discovered that room modes may be the issue. What is perceived as slow bass is in fact the lingering on of room modes, and not any mechanical slowness of the subwoofer speaker. The issue with the Quads is that they are dipoles, so they excite far fewer roommodes - that is why they are so clean. As a result the contrast with a monopole sub and the room modes it excites is all too obvious.
So I bought a DSpeaker Antimode 8033 room eq unit. The result was remarkable, and that for such a cheap and easy to install piece of kit. Bass was far better defined and also seemed to extend further down. The take home lesson is that room modes need to be addressed, and there are two ways to do this: multiple subs and room eq, preferably combined. So in your case perhaps an Audiokinesis Swarm or some other system with at least two subs, plus something like the Antimode.
You do not need external filters, unless you also want to high pass the Quads. That is really only useful if you need a louder sound. I have the Antimode connected to the speaker output of the amplifier, via an attenuating cable (the Antimode only takes line level input). From there to the line level input of the sub. No problems there at all.
As for your amplification, I am a bit concerned that you do not have enough power. The 2805s are a pretty inefficient speaker. My original amp with the ELS 57 had been the 2x45 watt Quad 303, but with the 2805s and in a bigger room that was not enough. It was fine at lower level and with not very dynamic music, but with the bigger symphonic repertoire the sound became a bit compressed and strident. So I bought a refurbished 2x140 watt Quad 606-2, for an indeed rather cleaner and more dynamic sound with demanding repertoire. This is essentially the same amplifier as the more recent incarnations, the Quad 909, the QSP and the new Artera. I am even considering getting the Quad QMP monoblocks (perhaps with the forthcoming DSpeaker X4 pre amp with even more eqalization options). However, my first new purchase will probably be a second PV1d, for even smoother bass.

Yes, I know there are (a very small number of) dipole subs. It was Gradient in Finland (not DSpeaker) who made them. However, they were huge and not particularly cheap. I am in Europe, so the GR Research offerings were not really an option. But yes indeed, if you are in North America and have a bit of space, they are interesting.

@willemj

Subwoofers are hard to do well and 8 inch woofers are rather small for that application. Room modes are certainly part of the usual problem. If you have a chance - try a JL subwoofer before buying another PV1D - you may find it integrates better. B&W make superb products but they are often bass heavy and resonant. An important factor in subwoofer design for audiophile high fidelity quality is group delay - hardly anyone measures or quotes this but I have seen measurements for JL and they are superb - measurements for many other subs I have seen tested were terrible (primarily because higher output is achievable at the expense of high group delay)