Pass x250.8 vs Boulder 850 vs Spectral DMA250

@George, yes the C2500 output impedance is 250 ohm (I just received the email from macintoshlabs).

I don't know the vintage of these and there are many many others to look at, so this is a bit of a guide only for you to do your own home work with.

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

Parrasound JC1 monoblocks

Ayre MX-R monoblock

Simaudio Moon Evolution W-8

Mark Levinson No.431

Mark Levinson No.436 monoblock

Classé CAM 350 monoblock

Classé Omega power amplifier

Most of the older big Krells, the ones with masses of heatsink are bi-polar (BJT) So is Dan Agostino's new Momentum and others are out there.

With your speakers stay away from Mosfet output and look at bi-polar (BJT) if you want the very best out of an amp that can drive that nasty bass load your speakers have.

Cheers George

Thanks George, you gave me a lot of material to study for :-)
Max

"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.

@Zd542 definitively yes. BJTs that George suggest are valid and with no doubt they potentially drive the SF at their best. But the sound pressure I can drive these speakers is for a normal listening room (50sqmt in this case) so the power request can be managed also by Bouldes or Pass.

If you know the SF Elipsa Red or Stradivari sound, they have a wonderful bass but it needs a lot of control.

First I was oriented to pure Class-A amplifiers but when I realized that most of them are monos -and I don't have space for two amps- I was looking elsewhere.
From the George list I guess the only amp I can ask for a test is the Ayre.

Sure I will test all initial three.