The problem you are running into
@Dude#1279435 is part of why I ditched the idea. That, and cost. This was my thread last year:
I repurposed my older Logitech kit but need something on my computer as the built in speakers are... lame? :) My searches keep me coming back to Audioengine powered speakers... either A2+ or A5+. Don't need to compete with my actual setup, just want better sound for my computer needs, thus am...
forums.audioholics.com
The best way to handle this is with a MiniDSP and REW to manage the interaction between the Speakers and Sub. The draw back, IMO, is the complex adapting circus that needs to take place. Oh... and did I comment on the COST already?
If you are seriously into this, then yes, it is worth it. You should find a Sub and Speaker Combo that will work well together, and then, frankly, you should spend on a very high end soundcard solution... external, internal... whatever you need to do to solve the problem.