I have both P15 and PSM-156 in my system, and I have tried quite a few combinations. I got the biggest improvement from plugging in MK2 into PSM-156. I also have M1200 amps in it too. This combo gives a much quieter background with greater focus.
However, adding more gears into PSM-156 the music becomes less engaging and less lifelike. I know other members here have switched to all Puritan PSM conditioners, and some did not like them at all. So, it is system dependent.
When you get your P15 back, you should plug everything back into it, and hopefully you will find P15 has its charm too.