jellyfish, I have those CDT coaxials. They are good coaxials that have good sq and midbass for a simple coaxial. BUT, it sounds like you may expect more bass out of them than they are capable of giving. One of my installs uses the CDT CL-6x's as front stage but the sub I use is an 8" sub. So they can keep up with a small sub, but a 15" or two 12"'s, I don't know. My CDT's CL's, Viper comps, and Crystal CSC's could do it with no problem.
I have mine crossed around 90-100hz with a 50x2 rms amp. In my vehicle, they sound very good like this. If I put them too low they start to distort. In this install, I'm willing to give up some midbass in order to have overall clarity. So what I'm saving is that they sound very good if you use them in their intended frequency and power range.