what can I get by with?

i play the kestral 2's with 60 wpc push pull. the k2's are apparently more efficient than the originals.. are you sure of sensitivity value.? if so, the obvious ? is how loud do you need them to play? do you have a decibel meter, if so just do the math. if it is truly 86db i doubt you will really get them singing the way you should but it depends on the source, room, etc. honestly i don't think 86 db. speakers are a decent choice for low powered amp even if it is the SET type. of course if you bought some hi $$$ SET you may pull it off but at that point you might as well get more eff. spkrs. interesting question tho!

Conventional wisdom says no way with 300b. 89 dB with benign impedence and phase angle worked OK for easy material but began to be congested if you wanted to boogie a bit, in my house, in a 23 x 14 x 8 room.

How about some Omega Speakers? They are very efficient single driver speakers made for SET. Often seen here at A-gon.

What can you lose by trying them? As stated: It all depends on your room size and the volume you require to be happy.