Tube amp for a somewhat difficult load

I would think the Audio Research models would be worth looking into. Also Sonic Frontiers. Neither of these makes give you "tube sound" in the sense of soft sounding, they are supposed to be 'accurate' not 'tubey'.
You could try the 'V-12' from Cary, switchable ultralinear/triode and offering a decent amount of power.

Amps of power do exist, but they are VERY expensive. CAT amps, at about 100 watts pure class A( most unbelievable 100 watts I've heard); Atma-sphere has 2 models, MA1 and MA2, rated at 160 and 220 respactively. ARC has very powerful tube offerings. Jadis is a tube amp I have lived with and greatly enjoyed. VTL, Shoreline, are some others. Check used tube amps.

I have a pair of Thiel 3.6's (86 db and 4 ohms) with a pair of Levinson 20.6 100W mono's in a 13 x 24 x 9 room. While waiting for the factory to send upgraded circuit boards, I purchased a pair of Wright-Sound WPA 3.5 SET's for the interim. That's three and a half big ones of power. Bottom line - with the 20.6's a comfortable volume level on the CAT with classical/instrumental/jazz is the "ten-o-clock" position on the volume knob. With the WPA 3.5's the same level was perceived at the "two-o-clock" position. Even orchestral music was reproduced without any strain whatsoever. To say I was surprised is an understatement. But note: most SET output transformers are jumped for 4,8 and 16 ohms. Can't really say what a 6-ohm Z would do.

Wolcott Presence monoblocks should work well also.

If you are looking for great tube sound :

JADIS JA 100/250 (the newest model replace JA 80/200) period.

JADIS JA 80/200, VTL 750 ref, BAT VK150SE, or CARY V12 Mono are my second choice !!

Get rid of your SS right away and you'll find the MUSIC should be !!

Good luck !!