amp I've used including an old 30 wpc stereo receiver.
For what its worth, the STs are nominally 4 ohms, but they are also a very easy load. I've driven mine with an old Denon HT receiver that does 70 wpc. Salk says any amp that delivers an honest 50 wpc will drive them. I have heard STs nicely driven by a 30 wpc tube amp. So even though I now use a 200 wpc amp for them, STs are an easy load than most any HT receiver can drive.
This is an impedance curve for the STs, red is impedance in ohms and green is impedance phase in degrees. It can be found here. They go as low as 4 ohms, but never lower.
Realize that the SongCenter and SSII are essentially identical.
The QWT transmission line design primarily affects the bass performance of the ST. The main feature of Salk sound that I like so much, a crisp clean present midrange with excellent dispersion and imaging comes in all the Song series models. It is largely a result of midrange performance due to the crossover design, and that is independent of how the cabinet handles bass.
The ST and the SSII are MTM designs, and they do have a differing dispersion patterns and presentation than the SSI. But they all share a strong family resemblance.