PerroNegro,
I will be building a pair of the Alerions this summer and will use them with the Stereomour that I will most likely build for 45/2a3 operation, and I also have a pair of Fi 2a3 monos which sound really great with my other speakers which are roughly the same sensitivity as the Alerions, though they do have an 8 inch driver and my room is small -- about 150 SF. You don't say how large your room is, and you might be able to get mileage out of a 45 by limiting the lower freq cutoff of the amp and using a pair of subs. Of course that's more money, more complexity and as Hank mentioned, matching a sub to a horn isn't easy, though easier with back horns than front loaded.
Another option to consider would be to build the paramounts as 300b amps but wire the filaments for 2.5v and use the high-power 2a3 tubes such as the JJ, Sophia, and I think the solid plate EML will also handle that power, but I wouldn't want to be the guinea pig on that test. I would probably take this route myself if I had the money as the alerions will be in the living room, not in my office/listening room.
HTH,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)