Best ARC Solid State power to match ARC Ref 5 Pre

If I wanted tubes AR would be my first choice but for SS I think there are better choices from other firms. I think my CJ 350 would fill your description better than anything AR has produced in SS and amps from Krell, Gamut, Musical Fidelity, McCormick etc would also be good choices. Tube sound with SS power is a hard thing to achieve; some feel it has never been done.

Audio Research makes some of the best solid state amplifiers out there. Unfortunately their solid state amplifiers do not receive the credit they deserve because Audio Research is primarily known for their tube electronics. That said , I can highly recommend an Audio Research D400 MKII. The D400 MKII will mate perfectly with your Reference 5 preamp and it will have plenty of grunt for your Sophias. The later version would be the D225 or the Reference 150 depending on your budget. I used a D400 MKII with my ESP Concert Grands and I can tell you from experience it is a terrific amplifier. I would put it in the same league as Levinson, Krell and Threshold and much better than the others mentioned here.

It would be nice if folks here could recommend something other than what they are currently using in their system.

The general thinking is that the D400MkII and the 100.2 are two of the best solid state amps from ARC. The 400II has more authority and maybe a tad more extension and it is clearly bigger and heavier in case that matters. The 100.2 is slightly more "rounded" sounding but it's probably mostly a matter of which will provide the best synergy with your system and room. I haven't heard a Ref 5 so I don't know it's characteristics (or which amp would best compliment it); if you might be power limited the 400II is probably the choice; if your system/room has any edginess the 100.2 might be the better choice. Both amps are likely to hold their value well in the used market; if you buy one and it doesn't trip your trigger you could probably resell it and try the other and be out not much more than the shipping cost on the amp you re-market.

"It would be nice if folks here could recommend something other than what they are currently using in their system. "

Why don't we all start recommending things we have never actually heard?

Okay rrog, here is one that will satisfy you.

My friend, with an ARC Ref. 5, uses the 220 wpc LAMM M2.2 (hybrid)
monoblocks. These amps are fantastic, and are among the best couple
of amps I've ever heard. (Fyi, they only have 1 6922 tube in each amp,
so they are almost solid state.) I should point out that he runs them
balanced, (i.e. with XLRs).

The sound is very refined, and incredibly transparent, and the bass
response is tight, fast and deep.

My two cents worth.