I am a very content owner of the Solaris, which drives very well my Audeze LCD-4 and Abyss 1266 CC Phi, phones which I suppose are not easier to handle than the Susvara (which I never have listened to though, only had HE-1000, now sold ).
For those phones I have my volume knob only at 9-10 o´clock in the OTL mode.
Moreover, I recently got the STAX 009S which the Solaris also drives perfectly in the Stax+gain mode,which for the same volume I turn the knob to about 13 o´clock
I have compared the Solaris directly to Stax SRM727 amp, and the Solaris makes a very good job, if not better!
Solaris has lots of Power and is a glowing Beauty to that!
Grüsse aus Schweden ! Olle