Biggest bang: add Behringer or replace CDP?

I second the CDP.

It's not even a contest. The Behringer will have much more of a positive improvement on your sound. Also the dac in it is the AKM 4393 which isn't bad. I seriously doubt the other two posters have used one. In my dedicated and fully treated listening room which is LEDE with 4 large bass traps the improvement is MAJOR. My system consists of:
Krell KRC-3 preamp
Sony DVD player
Bel Canto Dac 1
Vmps RM 40 speakers and a Vmps sub
If you do some research on room acoustics you'll learn that a room response of plus or minus 15 db is average. This is a huge swing in amplitude. Once this is tamed the realism and enjoyjment is much higher. No more weird effects from harmonics whose amplitude is much higher or lower than the base frequency. This is stuff that will hit you immediately. Use the dac in the DEQ 2496 and then get a better dac down the road if you like. I used the stock dac for a while and enjoyed music immensely and have read others do the same. To put it in perspective, one newer cd player with a better analogue section and dac couldn't possibly match what a DEQ 2496 can do. For 300 including a microphone I'd rate it as the bargain of the decade. Do some more research on room acoustics.

I don't think that your question is easy to answer not having heard your room. I have owned a cd63se cd player and you can do much, much better now. The Marantz isn't a bad player, but you will be amazed at how much better cd playback can sound with a better digital rig. That said, most listeners severely underestimate the effect a rooms' characteristics have on what they hear. In addition to the problems Warnerwh addresses, problems that diffusion & echo absorbtion address also severely hamper your systems' potential. I haven't heard the DEQ2496, but what I have read makes it sound like a bargain improvement. My only concern about the Behringer is the fact that it's a cheap, mass produced item from a company with a spotty at best reputation for reliability and customer service. When I researched the DEQ2496 and other similar products, I was taken aback by the reliabilty problems Behringer has.

I hadn't noticed your 1200 dollar budget. With the extra money you could get a Bel Canto Dac 2, Benchmark or any number of other dacs that are truly excellent. That combination is you best bet in my opinion. Even if you're very lucky to have a room that is much better than average, say plus or minus 10db swings(this would be rare), I think my suggestion would offer you the most bang per buck. With the Marantz as a transport and a truly excellent dac you get a DEQ 2496 as a bonus.

The DEQ 2496 will make an amazing improvement to any system because, as your equipment becomes better and better the listening room becomes the dominant flaw. Even if you don't pass your signal through it, and use it only as a RTA, it's worth the $300. No reliability problem with my two units.