What Unusual SET tubes you've tried & liked?

Cheers Audiofeil. My amp builder has similar tastes. You hit the nail on the head and here's the method behind my madness. I can run my AKG K 702 headphones with out the problem of limited headroom that speakers present for these musical tubes. Do you run 2a3 single plates?

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!

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

They are exceedingly uncommon/hard to find but have you considered a PX25?  Similar to a 2A3, Art Audio produced a 5w power amp for years using this tube and it is an interesting middle ground between a 300B and 2A3.