Many 4 Channel Amplifiers have a set of low pass filter outputs to drive a single channel or dual channel sub-woofer power amplifier. Those could be used as input for a Sub amplifier.
You configure the 4 channels as high pass and use the low pass outputs to run the Sub Amplifier.
Some 6 channel amplifiers like my McIntosh MC4000M have the high pass/low pass with amplifiers for all 6 drivers in one chassis. My previous MC440M had the same arrangement, just lower power.
Make sure that you build a decent power infrastructure & cooling around the amplifiers for best performance and stability.
The MC4000M (100Wx4 @ 4Ohms, 300Wx2 @ 4Ohms all rated at 12V) can demand 144 Amps. It is a very conservatively rated product.
I created a Local Power Center near the MC4000M Amplifier Power terminals. It's overkill, (I'm an Engineer, and yes, I have it modeled in LT-SPICE - Very Geeky, I know), but, I have:
Short, 0 Gauge, 1000+ Conductor Power wiring, Same for the ground return for the Local Power Center
1 ea, Maxwell Technologies B-MOD Energy, 58 Farad, 15V, Low ESR (10 milli-Ohms) Capacitor, it provides 4kJ (4000W/seconds) mimimum Energy
2 ea, Kinetic Audio HC600 12V AGM Deep Cycle, Low ESR Batteries in parallel
1 ea, 10,000uF 35V mid-ESR capacitor to damp out the resonances of the charge current between the 58F Capacitor and Batteries.
It's all wired to deliver a low impedance source to the amplifier.
The design supplies both constant Power (Battery), and Constant Peak energy capacity (Capacitor), locally for the system. The idea is to supply a very low source impedance over frequency (Middlebrook criteria) to guarantee the energy is available on demand. Some devices like 10X lower source impedance than the user of that power from DC to 50kHz. Mine is in that range. You can actually damage a power amplifier by "starving" it of this energy as it requires more current to run becuase the voltage is low where it needs it.
The MC4000M has 4 separate Power Supplies. There are filters between each of the 4 supplies (Series Inductor and 2 ea 2200uF for each supply). It's a very well designed amplifier.
I use the path back to car battery (an Optima Red Top), really as a charging source for the Local Power Center in the trunk. I even sized the cable back to the car battery to make it a higher resistance path (4 AWG) to force the Local Power Center to supply the majority of the energy locally. This also protects my Alternator (I have been able to remain with a stock Alternator), as the cable will not support the high current path back to the Alternator/Battery. Only the delta voltage appears across the cable.
The rest of the system is a McIntosh MX-4000 Head Unit, the Empeg Rio (2 ea 250GB drives) of course, Boston Pro Components in the doors, Boston 3 way 6X9s in the rear deck, and 2 Infinity Kappa 1220W (more efficient than the newer ones - hard to find these) 12 inch drivers in a partitioned sealed box.
The system sounds awesome. Doesn't matter what is played from Carpenters through Led Zepplin. Needless to say, my headlights do not dim at high volumes!
In SI, a little termination and attention to layout goes a long way. In EMC, without SI, you'll spend 80% of the effort on the last 3dB.