What Unusual SET tubes you've tried & liked?

Another good tube for SE use is the 46. Don Garber of Fi made some amps using it (even though it is a tetrode). 

I have a Will Vincent SET 45. Real nice amp! But its 1.75 wpc limits its useability with most speakers. My Sanei 560A with 2A3's is a bit better (3.5 wpc). Last, I have a custom-built SE amp using the 6BG6 tetrode. 6 wpc, the pair of output tubes are driven by a 12AT7 (each plate drives one output tube). Very good SQ, indeed! And 6BG6's are quite affordable compared to 2A3's and 300B's!