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!
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