As someone who has been at the audio game since about 1976 when I had a pair of small Infinity speakers, a Mac amp, and a Phase Linear preamp, I can say that, like a lot of you, there has been a lot of gear in and out of my life.
I started years ago with the PPP (premier power plant) and it made a noticeable improvement in the sound over using nothing at all. Going to the P5 in 2012 was a nice move up, and the P10 in 2016 was noticeably better in all areas over the P5. With the P15 I expected a nice jump like the P10 was over the P5. WRONG. It was not another move up in sound, it was transformational. Audio memory is a very iffy thing because there are so many factors that come into play, but I do not recall any change I have made in years, not even moving up in speakers, that has done what the P15 has done. The sound stage is so much bigger and so three dimensional, the detail is through the roof better without being brighter or more artificially detailed like some amps and preamps might do. It is both more nuanced and more powerful. The excitement of the music is so much more palpable and the sheer joy of listening to Jen Chapin and the Rosetta Trio, Fourplay, Fleetwood Mac, Mark Knopfler, Brahms, and the others that have been played since I got this new beast is way better. It took about 48 hours for it really to get going, but boy oh boy what it has done to my system is amazing/transformational. I have not heard my system with the BHK mono blocks (have the BHK 250), so what I am going to say now may seem a bit of hyperbole, but if I had to choose between the mono blocks and no P15, or one 250 amp and the P15, I know what I would do.
After 40 years of playing in this field, I am convinced that the two most important components of a truly first-class audio system are a great listening space with proper dimensions, room treatments and speaker setup, and a power regenerator from PS Audio. For those who are reading this and say that this is not in the budget, go buy a used P5 or P10 from Audiogon (as I write this on August 6, 2018, there are a few listed). It will be the best investment in your audio (and video) system that you can make. And start saving your money so that in a few years, you can sell the P10, and get one of these magnificent creatures. You will not be sorry.
Kent Tager aka the Clothes Pen.
Thiel 3.7 speakers
BHK preamp and 250 amp
DMP player and DSD DAC