Headphone Advice for a newbie

Hi, firstly I confess that I am basically new to the headphone world, mainly due to new neighbours who are less accommodating to my love of playing loud heavy metal music, & secondly (& i must say more importantly, as she's sat next to me) my wife being sick of listening to loud heavy metal music. I have bought B&W P5 & C5 for use with my iPhone et al, but now due to needing to listen via hi fi (Arcam AVP700 & Cyrus CD8SE with PSXR) to a set of cans, currently my P5s. I had decided to buy some dedicated hi fi cans such as Sennheiser hd650 or Grado SR325is that were recommended by the hifi shop I have used for the last 20 years, however, when Joe (sales assistant at said hifi centre) who has known me for these 20 years, rang me, stating that his colleague had given me wrong advice, he has possibly changed my thinking. He recommended that I buy a dedicated Headphone amp as he felt that the P5s were good enough to enjoy as a hifi headphone set, & that the headphone amp would bring their performance to a different level. He suggested the Pro-ject amp, which is currently not in stock, but by memory thinks it is £200 mark. When checking on internet I can only find the Headbox mk2 (£95) or the SE version (£195) with 2 headphone outputs. I will only use one set of headphones at any one time, so assume that the SE is more due to the extra output, so will only need the mk2. Considering my equipment ( I will use outputs from my CD player straight into the headphone amp) will this amp suffice or will I be better spending more & can go unto £250, & if so any suggestions? Joe has stated that he will loan me the Pro-ject when back in stock, which I will obviously do before buying, but they only stock the Pro-ject. Any advice will be greatly received & accepted. Cheers