Hi the_crow.
If looking at the Sonus Faber Stradivari, as Stereophile tests show (below), your going to need an amp that can deliver good current, a good designed solid state amp that has bi-polar output stage (not mosfet or tube) is the typical way to go, one that can almost double it’s wattage for each halving of impedance.
How many watts into 8ohms is not the problem (as they are 93db efficient) so a 50watter into 8ohms will do, but one that can keep almost doubling that 50w down to 2ohms would be nice.
Stereophile Test on them states.
"its impedance (fig.1) dropping to 3 ohms and below for much of the upper bass and midrange. And the combination of 4.4 ohms impedance magnitude and –48 capacitive phase angle at 55Hz will give lesser amplifiers conniptions ( a fit of rage or hysterics)"
Out of the two amps you mention, look at the specs and see which one goes closer to doubling it’s wattage for each halving of impedance, hopefully from 8ohms to 4ohms to 2ohms. EG: 100w to 200w to 400w
Cheers George