I have the Coincident Frankenstein 300b MkII, but they are not yet hooked up to my Volti Rival speakers which are 100db efficient. I choose the 300b tube because that’s the tube most seem found of. I choose the Coincident Frankenstein because of the many very positive reviews and it’s price point seems to be a sonic bargain (selling direct helps). I choose the Volti Rival because I wanted a neutral, linear, highly efficient speaker to use with fleas watt amps.
+1 @lak, @charles1dad is a knowledgeable owner/user of the Coincident Frankenstein MkII. IIRC @charles1dad did not like the upgrade to MkIII, but I’m debating to get mine upgraded to mkIV.
Google "coincident frankenstein reviews"