"Here are just some that look to have bi-polar output transistors as they seem to come close to doubling wattage for every halving of load impedance from 8ohm 4ohm 2ohm.
I dug these out of Stereophile's test results they seem to do the current needed for the heavy load in the bass your speakers have, but as for the sound it's up to you to decide"
Bipolar transistors are not the only option. There's plenty of amps that don't use bipolars that will be able to drive the op's speakers. An amp is the sum of its parts put together by a designer that knows what they are doing. I don't know if you have ever heard any of the 3 amps the OP mentioned, but the Pass and Boulder will will drive the SF's just as well as the Spectral, if not better.
I dug these out of Stereophile's test results they seem to do the current needed for the heavy load in the bass your speakers have, but as for the sound it's up to you to decide"
Bipolar transistors are not the only option. There's plenty of amps that don't use bipolars that will be able to drive the op's speakers. An amp is the sum of its parts put together by a designer that knows what they are doing. I don't know if you have ever heard any of the 3 amps the OP mentioned, but the Pass and Boulder will will drive the SF's just as well as the Spectral, if not better.