["lanx0003 I sort of shy away from adding a sub, primarily because I do not believe a separated sub could easily match the speed and texture of the main speakers. I had tried at least three times (SVS and REL) but none of the sub. I have tried provide me satisfactory results."]
I’m unaware of any subwoofer that claims measurably superior speed. Since the speaker location is typically the worst location for a subwoofer your dissatisfaction with that subwoofer and even with the sub-bass speaker is predictably understandable.
Consider the majority of full range statement speaker demonstrations are usually accompanied by determined room positioned and system tuned subwoofers providing an indistinguishably low frequency presentation.