... sorry to butt in.. but are you trying to put more speakers (drivers) on the sub output on your soundcard? or just pluggin stuff in? i see 2 scenarios:
1.)you can use lots of nice 2.1 systems, use the front and rear jacks.. and add a separate amp on the sub/center to output to a sub and center speaker cabinet(s).. this means you have control over the type of sound you make: party boom-boom or movie watching..
2.)take all your stuff out of their boxes(amps, speakers) and redo everything.. there is a nice thing you can do to get more from a small amp.. first.. look at the size of your speakers and their build type, buy pairs of them.. for instance.. if you use 2 speakers in parallel instead of one, you get more sound, at the expence of half the impedance... so, you use 2 speakers in parallel in series with 2 identical in parallel.. so, you get higher sensitivity and the same impedance.. this way, your amp will stay in normal working conditions. Works better with 8 identical speakers.. wire them in such a way the impedance stays the same, but the sensitivity goes up by +9db (double the sound at the same wattage)
so, if you wanna use the same amps you have.. buy lots more speakers.. then make nice towers for everything.. remember to match the outputs for all channels (gonna go expensive for subs...) have fun and google