I have the AKG k701's, they aren't a difficult pair of cans to drive, Id assume the 601's aren't much different in that reguard. If you can swing it I'd give the 701's a look also, Headphone.com (HeadRoom) had them for a fair price, you have to call them for pricing, or even consider a well-taken-care-of used pair.
I've only heard the 880's a couple times so any input I'd have on them would be about worthless, but in my opinion the k701's are phenominal if you want an open sounding/transparent and very detailed set of cans, they just don't sound like headphones. (in a good way)
I don't know about the 880's but my HD-650's want power, they don't truly shine until they're fed juice, with and without an amp is night and day. With the k701's the improvement not as drastic, they seem to benefit more from a cleaner amp (better detail, soundstage), not as due to them being under-powered. I'll use my AKG's on my Marantz's aux jack or DAT deck's jack, I won't bother with the 650's.
A quality amp should be a nice improvement to most stock (if not all) reciever auxillary headphone jacks, but if you want to save some money now invest in a nice set of cans now and pick up an amp down the road when you have the extra cheese. With the AKG's you'd probably be able to really enjoy them without an amp, and leave some room for improvement later.