I have listened to the Pro-ject MKIISE - in an audition with the £400 Musical Fidelity M1 HPA. The latter is very warm-sounding, "oak barrell" I described it when I heard it. The Pro-ject is fast and bright - wouldn't suit certain headphones, though from what I heard of the P5's in an apple store, they're pretty warm so would be ok and metal would sound good. Having said that, the P5's are intended to be used with a portable device, a headphone amp is simply a luxury. If anything, a FiiO E5 would do just as well; I feel this guy is trying to sell you something you don't strictly need, unless you buy harder to drive headphones.
By the way, I use and really like the NJC Monitor Audio II, made by one-man operation Nick Clarke in Cornwall. I bought it in the summer, about £270 (roughly same as Graham Slee Novo but I have heard it equals or probably beats that amp). Thing is, just after I bought it, he added a new circuit board, and a line out stage so it's also a pre, and added a hundred quid to the price! No longer quite the giant killer it was, but I recommend you look at his site.