Pass x250.8 vs Boulder 850 vs Spectral DMA250

The McIntosh preamp is a really weird match for any of the amps that you list. I wouldn't buy any one of them unless I could try it in the system first.

I agree that the Spectral may not be the best match because of the Spectral preamp issue. I haven't heard a Boulder amp in years so I'm really not sure if that would work well. I will say that I think they were a little overpriced for what they give you. I don't know how different their newer models sound, though. And that leaves the Pass. Its a great amp, but if you look at how its designed, its very different compared to your Mac preamp. So much so, that I think the 2 pieces would be working against each other. For example, Mac believes in using a lot of neg feedback, where Pass doesn't use any. Pass believes in using a very simple design (very few components in the signal path). McIntosh is just the opposite. Looking at it that way, its easy to see potential conflicts. I'll use one of my systems for another example. The components are made up of several different brands: Ayre, Wadia, Aesthetix and Vandersteen. Even though I'm using 4 different brands, they work well together because they are designed by companies that think in a similar way. If I took your Mac preamp and put it in my system, it wouldn't allow the other components to sound they're best. If I took the Pass amp and put that in my system, it would be a much better match. It wouldn't take the fully balanced, 0 feedback, in phase signal coming from my Wadia and Aesthetic front end and turn it into something else. It would retain those properties as it continued to the speakers. The Mac preamp would introduce feedback to the signal. Feedback also alters phase. So just putting 1 odd component in a system can have a serious impact on SQ.

"It never gave me the emotion I want in music!!"

I thought you want 3D? No more? Now you want emotion? You keep changing.

"@Zd542: I partially agree with you about the possible degrading of SQ using different solutions due to a couple of facts I experimented in these days: the C2500 sound quality is very high compared to some friend's SS preamps and it is (suprisingly for me) neutral, and I'm not a fan of Mc gear.
Second, a good match, in my experience, can be completely unexpected as this little class A ss amp that changed my vision, even with its limits, and convinced to me to try the solid state way without Mc output autoformers (slam, nuances, etc.)
I'm coming from a Vitus RI-100 it was a good integrated amp but not so fast and with a little roll-off on bass area."

I wasn't suggesting that you would have a bad match using your McIntosh preamp. I don't see your system sounding bad because of it. I think it may not be the best match to some of the amps you are looking at. Also, the preamp has a huge impact on the overall sound of the system. All the pieces in the system have to work together to get the best possible sound.

"08-19-14: Maz65
@Bo1972 It is not a mistery that Franco Serblin (r.i.p.) tested its own products, in the first instance, with Spectral, his personal setup, and (maybe not confirmed) Krell stuff."

Speaker manufacturers test their speakers with a lot of different amps. They have to make sure the speakers work with a large variety of products. Aside from Krell and Spectral, your speakers were probably tested with another 10-15 amps. At least.

Bo's right about the Spectral amps. They do have a unique sound. You may like it, but there's no way I would buy them without listening to them first. Also, with the Pass amp, you can't put it into the same class as other mosfet amps. Pass is the best guy there is with mosfets. He gets them sounding just as clean as most bipolars. Another amp you may want to look at is Ayre. They do use bipolars, but they don't sound like anything else. For solid state, Ayre is my favorite. But whatever you decide to do, you need to listen for yourself. Going by reviews and opinions is very risky.

"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.

"For the rest I agree with you, note that MIT and Spectral is often used in Italy because the dealer is the same ;-) But now we are OT..."

It probably goes a step further. Spectral usually makes the dealer use MIT or they won't be allowed to sell their products. I think that's how they do it here in the US. Spectral forces the dealers to use MIT. A few years ago when I went to a dealer to demo Spectral, I brought some of my gear into the store to see how well it would match with what I already had. The dealer got mad at me. When he went away to help other customers, I hooked up some Tara and AQ speaker cables that I brought with me. They were both better than the MIT. lol. When the dealer saw my cables his face turned bright red. I don't know if he was mad because I changed the cables without asking or he was mad that they sound better than his MIT. He chased me out of the store. lol. This probably happens to Bo all the time.