I'm kind of assuming that the unpleasantness you're hearing is either cone distortion or port noise.

I like Ken's idea about verifying the source of the noise (cone v. port, both speakers?).

You either have a defective product (somewhat unlikely, especially if they sound excellent up to a certain volume) or you're feeding the speakers a highly distorted signal.

The XPA-3 might very well solve the problem, if it tolerates a wider range of input voltage than the Luxman (quite possible). However, I still suspect something wonky with the sound card; to really screw things up requires a computer \:\) I would try to find (or borrow to test) a solution that offloads the audio processing into the component realm; just send the digital audio (either optical or coaxial) from your computer to a receiver or other processor and control it from there.

The key to solving your problem is to simplify the path from the signal to the speakers, and to remove the sound card from that equation to ascertain whether it is the culprit.

bibere usque ad hilaritatem