Received the P1u from Todd today and it is a beauty. I actually might like the brushed silver face more than the original P1's champagne gold. The P1 is about one inch wider, but the other dimensions are very close between the two units. Obviously, the P1u has 2 headphone outs instead of 3, but it does sport a loop out that is nice. The unit comes with plastic caps on all the inputs and the outputs, which I left on except for the XLR ins. The P1u feels much like the tank that the P1 is.
I set them up stacked on top of each other, and they are both plugged into a Shunyata Hydra 8 power conditioner with Crystal Cable Reference PCs. They are receiving identical inputs from the Amarra Model 4, which has 8 channels of XLR output. The cable, I believe, is a Mogami wire product, and it breaks out the 8 channels from a DB25 connector. Other than volume matching, there shouldn't really be any variables going back and forth between the two siblings.
I have just started listening and have tried a number of songs of different varieties, using an Audio Technica AD2000 and a Sennheiser HD800. Without going into detail, which I cannot really offer as yet, I would say that the two units sound more similar than different. I hope to spend some quality time with both of them to really see what I can hear that distinguishes them. From what I can tell early on, the P1u easily steps into its big brother's shoes.
Here are some pics, and it is particularly difficult to photograph these shiny beasts at night so they are nothing special.