Indeed, I learned a lot from the work by Geddes and Toole et al. Years ago, having bought my first subwoofer (a B&W PV1d) to complement my Quad 2805 electrostats, I was quite disappointed by the woolly sound (were people really right that electrostats and subs are an impossible combination?). When I turned down the sub's volume the woolllyness disappeared, but having a sub at such low levels seemed pretty pointless. So I started reading, and discovered room modes. I bought an Antimode 8033 dsp room eq unit, and the sound immediately cleared up, and integration was no longer an issue. However, this only worked for one listening position, so I added a second and then a third subwoofer, and I changed to MSO and a miniDSP 2x4HD to fully benefit form the multiple subs across a large listening space. I also added a high pass filter. MSO is functionally the same as the fancy software by Toole/Harman or Geddes. It optimizes response over a wide area by measuring and equalizing the subs individually for multiple listening positions. It is not easy to implement, and I still have some way to go, but already I am very impressed.
By the time you use multiple subwoofers, and more so the more subs you are using, their extension becomes less of an issue. Each sub adds another 3-6 dB output, so there will be less compression. My biggest sub is a SB2000 located in a corner behind one of the main speakers, and between them my three subs have a healthy flat output to about 16 Hz, in a large room.
As for subwoofer quality, I have the impression that my B&W PV1d with its two opposed drivers (in the back of the room) sounds a bit cleaner than either my SB2000 or my little Kef Kube 8b on a side wall. I don't know whether that is just my imagination or not. HSU subs are not for sale in Europe, so I have no idea. What I do know is that e.g. Toole advises not to mix ported and sealed subwoofers. You may want to investigate Arendal subs as well. Reviews (with measurements) are very positive. Unfortunately they have gone up quite a bit in price. That was one reason not to buy one when I bought my last sub (I could get a SVS SB2000 classic for just 600 euros), the other reason was that the single driver models have the driver on the side, and that would not have worked in the location that I had for that sub.
By and large I think the way you implement the subwoofer system is more important than the choice of the subs themselves: how many subs, how are they equalized, do you use a high pass filter for the main speakers, do you also equalize the range up to about 250 on the main speakers?