Thank you, appreciate it. I sent a note to Rythmic and asked the question about the impedance matching. On paper, a tube preamp - mine is an Aric Audio Unlimited with 600Ohm (@ 1KHz) output impedance, should be a very poor match driving such low input impedance amplifier. The fact that you're driving two subwoofers, I assume from the second parallel output of the preamp, and probably driving another amplifier from the other output, even if its a tube amplifier with normally high input impedance, should "theoretically" pose issues.
The fact that you're not experiencing any sound quality issues is an indication of how theory and measurements translate into real life.
I saw the F8 and it looks very interesting, especially given their size. One could use them as the "stands" for the SAMs given they have almost the same footprint....Of course with proper isolation and decoupling.