The suggestion about the pre-amp output wouldn't appear to be part of the explanation either(the lower impedance setting affects only the internal amplifier in the 663 and has nothing to do with the pre-amp section). The mystery remains.
I realize what you're saying makes theoretical sense except I remember without a doubt empirically that one of the reasons I sent the xpa2 back was that I couldn't get much sound out of it at the lower impedance setting, whereas the higher impedance setting caused it to go into protect; not sure what the 663 is up to with their settings, that should not be the case with preamp output yet it seems to be so.
That is one of the reasons I felt compelled to post, it seemed to me that the weak link, for whatever unknown reason, might be the 663 and it was unfair to persist with the assumption that the xpa2 was the weak link without better clarity.
Alternatively, as Alan has suggested, the lack of subwoofer may have been the culprit, which I will test later and report my findings.
