Warren - I lack the direct comparison of SS2 vs SS3, but can share some relevant experience. I have one SS1 and two SS2s, which I have mixed and matched and moved around. First, my room is weird. It is my guitar-making and music studio and speaker workshop rolled into one. Its acoustics are good, but unusual, so YMMV. In the corner of a second floor residential-construction building with 12K cuft with open stairway connection at far corner to identical space below and smaller attic space above. Lots of volume. Playback is intelligible anywhere in the whole envelope. The studio walls enclose 3600 cuft in an L with front and left exterior walls solid and other walls porous - non-reflective with lots of diffusion. Virtually no room modes. Speakers in the assymetric L corner, listening in the 15’ wide top of L. It’s weird, but it works and presents neutral sound and smooth measurements.
In this context I have experimented with Thiel subwoofers.
SS1 between speakers and in solid corner (front left). Thiel PXO sums R&L channels. Dedicated XOs for PPoints, CS1.6, CS2.4 (used with my CS2.2s) and CS3.6. The PPs and 1.6 are substantially upgraded by deep bass. The 2.2s and 3.6s additional deep bass is nice, but not musically necessary for me. Same speakers with pair of SS2s. The musical experience is qualitatively improved with all speakers. The art of mixing includes collapsing to mono without cancellation, but that business is difficult and approximate at best, as Unsound noted. Note that using a pair of subwoofers driven by a single PXO was substantially inferior to using a pair of PXOs, one for each channel, which keeps the bass information of that channel with the speaker for that channel, as Unsound observed above.
Regarding replacing an SS2 with SS3, I believe the only reason would be if your space / loudness requirements are overloading the SS2. I don’t overload my SS1 in the corner (best coupling) at robust loudness levels into an effectively huge space. Any model SS's low frequency extension is lower than musical content for all practical purposes. My experience is that a pair of SS1s or SS2s with 2 PXOs provides a highly satisfying musical experience for 2 channel music. I plead ignorance of HT .1 channel.
Note that someone here previously reported a big step up by using Thiel’s Sub Integrator whereby you can tailor a sub to woofer crosspoint at a higher frequency and custom slopes. I want one, but haven’t seen any on the market. Perhaps your Marchand might provide that function. I consider all of these options as more satisfying than going to an SS3, UNLESS you have a very large room, listen at loud levels, and are running out of steam with the SS2. I vote that two SS2s in any placement driven by two separate PXOs would be worth an audition.
Enjoy,
Tom
