Thread: Amplifier/sub question...

I could use a little help with set-up on my system. Thanks in advance for any suggestions - sorry for the long post.

Hardware:
I have no head-unit, car pc with Soundblaster Live! 5.1 PCI card, two amplifiers - a 4 channel MTX and a 2 channel MTX, Polk Audio 6.5 component speakers in front, 6X9 stock speakers in back, two 10" MTX subs in back.

Connections:
So, the 4 channel amp runs the front and rear speakers via input from SB Live soundcard (green and black colored jacks respectively). After playing with the set-up in SB Live software and adjusting the gains, I was able to get good sound out of all four speakers.
I tried running the subwoofer output from the SB Live (orange colored jack) to the input RCA's on the 2 channel amp and then from the amp's speaker 1 and speaker 2 output to my two subs.
[SB Live orange jack --> phono jack cable --> RCA cable --> R&L RCA inputs on 2 channel amp --> speaker 1&2 (- and +) outputs --> sub 1&2 (- and +)]

Problem:
With the above set-up on the 2 channel amp I was getting no sub output. When I plugged in the jack to the orange connector on SB Live I got a thump, but then no sound from subs.

Troubleshooting:
I checked all the wiring on my subs - OK. The amp is powering up and looks to be working. I set up the SB software for the 5.1 setting and ran the test - still no sound from "center" speaker. Hooked different speakers to orange jack on SB Live (without amp)- got sound.

Partial Fix:
So, I saw a diagram online that prompted me to change the set-up. I ran a pair of RCA's from the "sum out" RCA jacks on the 4 channel amp to the "input" RCA's on the 2 channel amp. (So, in this set-up there is no input from SB Live orange jack.) I got both subs to work and they sounded fine.

Conclusion:
This may be how the system needs to be connected, but I thought folks were running sub input directly from the soundcard. I would rather have that set-up I think.

Questions:
1) So, is there a way with my set-up to run the subs with input coming from SB Live soundcard, not from the 4 channel amp's "sum out"?
2) Are there advantages or disadvantages to either set-up?
3) If not, and my set-up as mentioned above is correct:
a- Does anyone know what the settings in SB software should be (5.1 vs 4 speaker)?
b- Does anyone know what the cross-over settings on my MTX amps should be? The 2 channel has a switch that has three cross-over settings; "LP", "FR", and "HP"? The 4 channel front has cross-over settings of "FR" and "HP" and the 4 channel rear has cross-over settings of "LP", "FR", and "HP".