OK, I got the XP-15 in, and let it break in for a week. Initially, it seemed very detailed but also very bright and a bit hard in the treble. As it broke in, it did mellow a bit.
Relative to my PH7, it had well articulated bass...individual notes were clearly defined. And it was really quiet...the music seemed to emerge from a totally back background. And it is beautifully built, the casework is excellent. And the user has compete control over not only resistive loading, but also capacitance and gain. It would seem to let you shape the sound of any cartridge to your system.
But as my wife quickly pointed out, it seemed to really emphasize the treble over the midrange. The PH7 was simply more musical, beautiful in tone, and really fun to listen to. With the XP-15, the mind seemed to drift off, listening to an individual instrument, while with the PH7, you just soak up the really huge soundstage and absorb all of the music.
Having my wife come in, I set up the XP-15, and let her listen to a couple of records. Swapping in the PH7, it only took her 1 minute while repeating the 1st record to say 'you can stop now'.
Unfortunately, the XP-15 is not the solution to my little problem. So it went back in the box and was shipped out. A really interesting experiment!!