Biggest bang: add Behringer or replace CDP?

As you present yr case, it's better to improve the cdp. The Behringer is a useful tool -- but better adapted to other applications.

Maybe an Enjoy (written Ah Njoe or something) which is a modded marantz for ~1000 would be a good choice. There are certainly others.
Alternatively, an add-on device may serve -- such as a jitter reducer (this guymakes one that's reputedly good -- but I've never listened to it. It's expensive (but cheaper than a new 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.