Adjusting the EQ might help some, but using it to dial out some of your highs for slapping may affect your fingerstyle playing negatively and you may lose definition on notes. It sounds like a good compressor is what you're looking for. When you slap/pop, it creates some pretty wild peaks (volume spikes) in your sound. A compressor rolls off those peaks above a set point, smoothing out your sound rather than dialing out frequencies.
My experience with on-board compressors is that while they may be decent, you may be better off with a separate one. An "after-market" compressor will have more features and control which will let you sculpt your compression, rather than just the one knob that your D-bass has. I've tried a few different amps that have had on-board compression, and have never really liked it when compared to an actual out-board compressor.
I haven't tried it yet, but the new MXR Bass Compressor seems to be getting some good praise... you can control the attack and release on each note, which I suspect would be the key control that would benefit you the most. I just picked up a 3Leaf PWNZOR, which also has an Attack control. Very nice for controlling how fast the compressor defines the front of a note. I previously had (and still own) an EBS Multicomp... no Attack control (which I've really come to like with the PWNZOR), but still a good compressor. Not overly squishy and fairly subtle/transparent.
I tried my board without a compressor for a while, because like UncleJimmy, I felt that I wasn't quite as expressive as I could be. But with the recent purchase of the PWNZOR, I've reapproached compression, paid more attention to how I'm using it and realized that it can be a valuable tool for taming the peaks you're talking about and adding punch to my sound.
I'll point you to Bongomania, the resident TB compressor fanatic. He's got a couple of great pages up, which have helped me out quite a bit:
Compressor FAQ and
Compressor Reviews.
With a good compressor, you should be able to set the controls in a way that doesn't affect your fingerstyle playing, but helps control the slap stuff. I had to sit down with my PWNZOR to really figure that out, but now that I did... I'm glad I have a compressor back on my board.
5sg.