Well I can't recall having ever sold one
personally however I had one of them on demo in my high end audio store and used it all the time. When I challenged my audiophile friend, a recording engineer, to demonstrate his "ability" to hear differences between amps in a small token bet, we both agreed it was the best device to serve as the source signal, playing his hand picked music of choice he brought for the occasion. You can see it on the left in this image of the test rig we used:
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He sat in the sweet spot throughout the blind test and that's his knee you see (part of) on the pic's left edge.