I guess I don't get why this is being over-complicated.
RCA from source to A5+, then RCA out (full range) from A5+ to RCA in on sub... and use the subs xover to filter out the high frequencies.
That's the simplest way to do it, and there is no need for a new sound card. Plus, this opens up essentially any sub that has line level inputs as an option...
I get that it'd be nice to high pass filter the signal going to the A5+, and to have the xovers in the digital domain (for both the A5+ and sub) handled by a soundcard... but it's not necessary to do it that way. If this were to be used for movies/games with the PC as a source, I can see it being more important to high pass the A5+... but for mostly music in a small room, it's probably not as critical.
I say probably, because lets say there is an issue of exaggerated bass or boominess due to placement, and filtering out some of that low end could help alleviate that...depending where the problem frequencies are.