Let me know if I get this correct. You have a 4 channel amp and you want to drive the front speaker with one set of channels, and two subs with the second pair? With this, you want the loudest possible.
If that is the case below is what I would do:
Use two sets of RCA cables for the front and sub inputs into the amp. If the amp has a Xover, set them to the proper settings. Next, unplug the sub RCA's and get a CD you like that can be played loud. Adjust your amp gain all the way down, turn your deck's volume control up about 85%. The adjust your amp's gain until you can hear clipping. Back off the gain just a tad. Do the same with the sub (after unplugging the RCA's for the front), leaving the volume in the same place. This will yeild a system that will allow you to play all the speakers as loud as they can, without having one set fail before the other.
Another way to get more volume is to bridge the amp. This is where you combine the left and right channels on the amp to effectivly double the available power. If your amp can handle a 2 ohm bridged load, wire your speakers so that both positive speaker leads go to the right "+" and both the negative speaker leades go to the left "-". This will give you the most power to drive your subs. BUT YOUR AMP MIGHT NOT BE ABLE TO HANDLE IT, SO CHECK YOUR MANUAL. Also, cooling will become manditory because the amp will need to disapate a lot more heat.
Another way is to get more speakers.
Thats all for now,