I have experience with several two-channel Spendor/subs arrangements. One of my current set-ups is SP-1's (14" stands) with a Rel Strata III ($1,200 new, sealed 10" driver, 100w MOSFET amp) connected in augmentation (high level) mode (run SP1's full range and pick up feeds from amp outputs to speakon connection to sub) in a room about 11' wide, but vented with a doorway on one side 8' from the back wall and a double-wide entry 13' at the opposite end, w/ SP-1's 4' off back wall and 5.5' apart, and sub 1/3 way in along back wall and 1' off. I was making tweaks over several weeks before I hit my optimum response, but that wasn't too painful and now the bass is a seamless, humpfree fit w/ the SP-1's, giving a tight, texured, harmonically rich response down to 22Hz; the soundstage can give the impression of a space twice as wide and deep as this room is. Both upper rolloff starting point and output levels for the sub wound up lower than one would figure, perhaps to compensate in this case for the fact that the SP-1's load this room very effectively down to 43Hz. Amp is a tweaked McCormack DNA-1 or Aragon 4004/II; Placette passive pre (3 inputs); Muse disc player; ARC/Michel/Clearaudio phono; KCAG and Nordost ICs. I listen to all kinds of music.
In the same set-up, I tried for a couple of weeks this summer a pair of FL6's that I had acquired for a friend, and the combination also worked vey well, the sub's response set at appropriately higher rolloff and output levels. The character of the sound was quite close, although the SP-1 combination sounded both more extended and fuller, but that's a function of the speakers (the SP-1's do have warmer upper bass and lower midrange--what one friend always calls "boxey resonances"--and a supertweeter; the FL6's, true 2-ways w/ a 5.5" mid/bass driver, are slim and petite and don't load this room below 50Hz).
Finally, I tried Spendor 3/5's (28" heavy duty stands) that I acquired for my sister in the same set-up (augmentation mode), and the response again was excellent. I never quite finished experimenting with the rolloff and output levels before I had to turn over the 3/5's, but as one would figure, the rolloff setting was quite a bit higher (the 3/5's go down effectively to only 80Hz), yet the combination tempted me for a moment to replace my beloved, 20 year old SP-1's.
I've also listened extensively to SP-1's (and S8's) with a larger REL Storm III ($1,600 new, ported 10" driver, 150w MOSFET amp) at a friend's house in a much larger, longer room (augmentation mode), and again, the response was excellent, albeit limited in volume and dynamic range by the limits of the SP-1's in such a large space. Amps were Aragon Palladium monoblocks; Placette pre; Meridian disc player. The SP-1's were replaced with S8's and the sound and dynamics now adequately fill the space at performance levels. He's considering getting a second Storm III or replacing the single Storm III w/ a larger Studio or Stadium model.
Finally, when I set up my sister's 3/5's (also using a MF A3 integrated, with an A3 disc player), I acquired for her yet smaller space a REL 108E sub ($500 used), a very compact 11' box w/ an 8" driver (100w amp, sealed), also run in augmentation mode, and the combination is magical, down to an effective 32Hz, with plenty enough volume and dynamics in this small bedroom space.
In all cases, sounstaging and imaging were considerably enhanced, which is the greatest contribution the subs make.
I've tried subs from Hsu and M&K (w/ external x-over) with my SP-1's in the past but never got either one to work as well as the Strata III in augmentation mode. I never tried Spendor's own subs, which I don't believe have the augmentaion mode, nor Velodyne or V-S subs (may check the latter ones out). In augmentation mode, the system's dynamic range may be limited by that of the speakers, as is the case with the Spendors and Magnepans I've used. The RELs run in line level HT mode, too, of course, but would require some external crossover. RELs are pricier than some subs, but I've found them to be worth the money and work great with Spendors and Mags.
[I also use a REL Strata III ($800 demo) w/ Mag MG12QR's (McCormack DNA 2 amp, Placette passive pre, ARC CD3 player, ARC/VPI/Clearaudio phono, Monocle XL, KCAG), and it's quick and tight enough to keep up with the small Mags, giving enough dynamics overall and excellent bass into the 20's in a 13' x 16' room with an 8' kitchen alcove at the opposite end.]
Sorry for the long response, but perhaps it may prove useful.
