Replace headphones or add headphone amp

I have Sennheiser 535s which I got for $79 on closeout, and which sound pretty good to me (my reference are Spica Angelus speakers, which are inexpensive, but pretty transparent).

Obviously 580s ($169 on Amazon) or 600s ($225 here on Agon) are better headphones, but for the extra $150 I wonder if I would be better getting a creek headphone amp (or musical fidelity, or other). Currently I use the jack on a marantz CD63, and sometimes a portable CDP to drive the headphones.

So what do you think ? Better headphones or a headphone amp ? Thanks, in advance, for the advice.
I owned both the 580's and 600's, no comparison between the two. I used them with a Linn Classik, a portable CDP through a HeadRoom AirHead amp, and with a Grado headphone amp in my main rig. I would strongly suggest you upgrade to the 600's then decide if you need an outboard amp.

Check out It's Headroom's website. I'm got a Headroom Cosmic amp and Etyomtic 4S's. These ear buds are simply the best. They are great for mobile use, the absolute best for air travel since they do not let any airplane noise in are are the best headphones i've ever heard. The Etyomtic 4P version is almost a good as the 4S but does not need a headphone amp.
I also own both the Senn. 580 and 600s (I use the 580s) in an HT system sometimes and the 600s for music in a separate stereo system-- sources are Sonic Frontiers Line 1 and 2SE pre-amps. I'd suggest up-grading both source and headphones, but I agree with Jeff, go for the 600s first-- they're great. Cheers. Craig
Well, I've got a pair of the HD 600's and a pair of HD 45's (yes, that's right, bought with allowance money in the early 80's) and it's the difference between a porche and a little red wagon with one broken wheel, or possibly a big-wheel with a missing pedal. That said, the 600's sound a whole lot better driven with a Headroom Home amp than they do from the jack on the Marantz receiver, or the Yamaha tape cassett, or, for that matter, the Rogue 99 Magnum preamp....
thanks everyone. I need to rethink my budget.