You can try
this with your iPod Classic.
Though it's a bit bigger than HM603,but the PROTRACK will improve the sounding,you can get a more wider soundstage and a more detailed sound from this!
But still,the iPod-and-PROTRACK combo is as neutral as the normal iPod's sound.What the PROTRACK work is just strengthening your iPod.
This combo is working well with the Beyerdynamic DT990 PRO,at least the DT990 PRO won't sound bad.
And you can amp them through the phone-out jack as the normal DAP if you like.
Oh,I forgot!You can also use the PROTRACK as the normal recorders as well!Hmm...,the PROTRACK was made as a recorder for iPod at first step,the amp is just secondary.
Any way,I like this combo,but I thought they couldn't compare with the HP-P1,they didn't on the same price at all.
Is this an amp? It is very long though, I want something that is portable and could be strapped piggyback behind the ipod classic