Volume Control-Power Amp-Subwoofer

This is probably a strange question-
I have a NHT subwoofer that came with the separate integrated amplifer. I was wondering how I could use spare power amps with more power in its place. Would I have to buy a separate preamp? I could route it through my stereo preamp or home theatre receiver, but would not have the separate volume control I have now with the NAD amp. Is there a cheap (I mean really cheap:) way of doing what I want to do? The power amps are Marantz mono blocks and only have an on off switch so I would need something.
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Thanks Jab that does help. Now I will research crossovers. I have two mono blocks that I want to bridge and use for power if I can. From what I understand, I have to make sure the subs impedance isn't too low for the bridged amps.