You are dead right. The clarity of the 2805’s is in no small part due to their dipole character. Dipoles excite far fewer room modes, so bass is smoother, tighter, and ’faster’. So combining an ordinary subwoofer with a dipole like the Quads shows up the difference in a painful way. It is not that the sub is any worse than with other speakers, but the stats are so much cleaner.
When I decided a few years to add a subwoofer to my 2805s I did not know what I know now. After a lot of reading and a bit of listening (pretty pointless in the case of subs because you are effectively listenng to the room) I bought a B&W PV1d. It was universally praised as just about the best match for stats, and its looks matched our modern decor.
The result was disappointing, with woolly booming bass that clearly sounded different from the stats. The problem was reduced by lowering the crossover frequency, and by restricting the volume, but to a level where you might wonder why you had a sub at all. So when I read about room modes and the Antimode room eq system I bought an Antimode 8033 (cinema). This largely solved the problem: bass is tight, ’fast’, and without any booming. Since then I have read more, and concluded that multiple subs would improve the sound even more, and allow for equalization over a much wider area. A second PV1d is on the shopping list.
Knowing what I know now two dipole subs would have been an obvious choice (I did investigate that at the time, having heard about them). Unfortunately I do not live in the US, and there is not really anything suitable on the European market. Shipping a GR Research/Rythmik kit would be expensive, and I do not really trust myself building them. Also, the subs would still be pretty large compared to the PV1d. So not for me. Having heard the improvement from the Antimode room eq, I am pretty sure a second sub will cure my remaining issues. More power is also good, having recently experienced the benefits from more amplifier power (moving from a 2x45 watt Quad 303 to a 2x140 watt Quad 606-2). An extra 400 watts is not to be sniffed at.
So, from my experience, and if you can house them comfortably, I would indeed get those GR/Rhythmik dipole subs, but with the Antimode 8033. If you want something small, you could do worse than the PV1d (but in a pair, and with room eq.). The crossover setting I arrived at was 33 Hz with 4th order slope and zero degree phase.
