SF Extremas

As a previous and disappointed owner I would think your amp has enough power even though these speakers are notoriously demanding. But they are just not good speakers. I would have thought you could spend less and do much better with something like Merlin VSMs which use similar drivers and sound good. They just wont look as impressive.

need more power...at least 200 per channel into 8 ohms.

I owned the Extremas and really loved them for their rich, refined midrange, great dynamics and silky smooth and detailed top end. Very musical and involving speakers. The ARC 100 mkiii will drive them OK, but if you listen at higher levels (and that's what Extremas really like), you may find the dynamics somewhat compressed. To really open up they would need 150 - 200 good tube watts or even more if driven by solid state.