OP, if bass is what you're wanting, subwoofers are the best way to achieve this no doubt. I wouldn't go with a smaller sub. Typically it's nice to match subwoofers so that their response is the same (even though room nodes will destroy this, it's nice to have baseline)
I have two subs in both my systems and it's a massive improvement over having one. It evens out the frequency response.
Have you done any EQing/DSP for this system? Often times bass response issues are because of room nodes which can be corrected by dialing in the peaks in the response. It really is not difficult to run pink noise through an analyzer program like Room EQ and determine an appropriate EQ curve for your particular response (especially in the region below 200 hz!!)
Do NOT go down an equipment rabbit hole if you just need better bass. The problem is that the ideal positioning for mids and treble is almost never the ideal position for bass. If you can crossover less sub frequencies to your Dynaudios, you will also possibly hear an improvement from those as well because the sub frequencies aren't being handled by your amp, or the speaker woofers, or the internal crossovers. I've noticed that clarity improves on my system when I engage my subs even though technically the DSP adds some distortion, that distortion is more than made up for the fact that the tower cabinets aren't vibrating from sub bass and the components aren't having to handle the toughest material (sub 80 hz)
Best of luck! My vote is, go grab another SB 5000, and dial in that bass response to your satisfaction by using REQ and a room mic like a umik-1
