Pass Labs X250 or X350?

You could always bi-amp with the "better sounding" smaller amps.

The X-350 also uses zero feedback and the X-250 uses a little feedback in the design. The X250 is a good all aorund amp that can be used with box or other speaker types and the X-350 should only be used with box type speakers from what I was told.

I own the X-250 with B&W matrix 803s and it works great.

The X250 is a more recent design/build and has received its share of rave reviews. The X350 has reached a pinnacle of its own in the TAS recommended components listing. I think you have reached the point of diminishing returns in attempts to gather information. You are at the point where listening is the only thing that may provide you with the answers you are looking for. At least, this is the point at which I start pounding turf.

Jwin- I wouldn't doubt what the service engineer said, a dealer told me the same thing- he told me it was due to the amount of watts the amps produce into class A after a certian amount of draw they go to a class AB operation(and believe it or not I think the smaller amps produce more class A watts then there larger counterparts, as you noted). I am VERY familar with the SC-4(non-a) and they were a very easy load on any amp we used, that being said I would say the X250 would drive the Dunlavy's with aplomb. If you are looking to invest more money I would just buy more music or perhaps look at something else to upgrade i.e. another component, but it sounds like you have good taste in gear :) so maybe that's not an issue. Happy listening!