The DSP30 and Sub EQ are two different animals. The DSP30 is a parametric EQ solution (among other things but for this purpose, that is what we will consider) that operates only in the frequency domain and is a single point EQ device. The SubEQ operates in the time domain and is a multi-point room correction device. One of the many benefits of this multi-point, time-based solution is that is (among other things) quite effective at dealing with bass ringing/overhang that can really mess up the bass. And while removing single point peaks with the DSP 30 can reduce room ringing, it is experientially not as effective as the SubEQ. In fact, I did not even recognize what bass that clean sounded like until I installed the SubEQ. (I was running Audyssey Pro on my pre-pro and using the DSP30). I am not privy to how steep the filter capability of the SubEQ is by my uncorrected response only has a few peaks of about 5 db so that is not a concern of mine.
Do I think that DSP30 is a bad product? Not at all, particularly since I found mine used for a great price and as I noted earlier, still use it to provide push button options for having multiple bass level options. In fact, it has a lot of functionality that the SubEQ does not have but for my current speaker configuration, I do not need all of it. But, in my experience, it provides a less refined solution in my room than the SubEQ for correcting my four SubMersives.