So much for not picking up an amp anytime soon. I got myself a PPAv2 amp and based on the reviews it seems like a good DIY amp so I'm happy. Hopefully I don't get any temptations from the FS section anytime soon for another amp :)
I think the K601 has fallen behind the HD600 in terms of enjoyment for me. I really like the HD600 and it took me some time to truly appreciate them and I think they are pretty close to a W4 in headphone form which is what I wanted. The K601 offers a different signature and different signatures are always good. I was a bit concerned than the PPAv2 amp might be too bright with the K601 but luckily I didn't hear that when I plugged them into the amp earlier today. It was only two songs but they both passed. I think the real test is when I get my DT880 back. It was supposed to be this week but it got delayed until Monday which is fine.
I'll be interested in reading what you have to say after you receive your DT880's back. I don't think I will even try them, as that is just how happy I am with the K601's now. . .but still interested in your opinion none the less.