Pass x250.8 vs Boulder 850 vs Spectral DMA250

Thanks George, about input impedance I also see Pass x250.8 has input impedance 50k unbalanced and 100k balanced.
Bouder 850 have 50k balanced and 25k unbalanced, so it seems not working at its best with a tube preamp.
Very intresting.
Now I'm writing do McIntosh asking them the C2500 output impedance: I was not able to find this info elsewhere.

Maz65 look in your manual of the C-2500 under specs, it's 220ohm so no problem either.

As you still mention the Pass and the Boulder these are both Mosfet output amps, while they both are very good amps, for your speakers they are not going to drive them to their absolute potential. Because of the nasty phase/load they have in the bass, only a bi-ploar (BJT) output amp will do that.
And the only one that can that you mentioned is the Spectral.
But there are even better than this if you care to know, just say so and I'll do searching for you and post them up.

Cheers George

George, any contribution in that sense will be greatly appreciated by me. So please give your suggestions. I'm in the situation where I'm "free" to test almost everything.
Max

@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