Pros: Excellent sound, drives almost everything with authority, gorgeous to look at, balanced inputs.
Cons: Very expensive and not so great value compared to balanced amps that are available. Surprising choice of the cheap alps RK27 volume attenuator.
If you bought an headphone amp like you might buy a regular speaker amp, the P-1u is exactly what you'd get. Typically for Luxman, the build quality is excellent and beautiful to behold. While I didn't test the P-1u, nor its predecessor, the P-1, with anything exotic, every pair of headphones I've plugged in to it didn't give me pause at all to think about the capabilities of the amp in the slightest.
Comparing it to the Audio-gd Phoenix, the Luxman is revealed to be very slightly on the sweet side of neutral, with more authoritative bass, but with marginally less smoothness and detail. It also doesn't have the massively wide soundstage that the Phoenix balanced did with my HD-800s, but this is less of a surprise. More of a surprise was the cheap Alps RK27 volume attenuator, which doesn't seem fitting for such a high-end amp. It is not purely "straight wire with gain" but, again typically for Luxman, the amp you get when you don't want to have to ponder all the complexities, but have it "just work" and sound good regardless. To boot, you can plug it in to either a single-ended or balanced source, so compatibility is not an issue.
The only downside is the asking price, which at $3000 from TTVJ (around $1000 more than the Japanese retail price) is extremely serious amp money, more than the cost of quite a few balanced amps. This, however, might be moot if one never wants nor needs to re-cable a stock pair of headphones.
If you have they money to spend on such a high-end rig, but can't be bothered messing around trying to work out what will work best with your headphones, then this is the amp to go for.