Hi Mrrita,
I'm afraid I'm not familiar with Dynaudio 1.3s other than by reputation. However I own a REL Strata 3 and I've found it to be powerful enough and easy to integrate. Mine is currently supporting Spica Angelus speakers, which go quite deep but are very lean in the bass. Adding the REL made the music much more enjoyable with its crossover set at 30Hz. It's very fast and not noticeable as a separate subwoofer until it's switched off, when you really appreciate the difference. I haven't tried any other subwoofers, but the REL is good enough that I haven't felt the need to.
Good luck with your search