alpine d-class amps have a narrow frequency response, less than 200hz i think, so a normal line-out will do, as well as sub-out.. u say u have two subs, running off a mono amp, ie 1 left speaker outl, 1 right speaker out.. Explain how u have wired the subs.. are they in series/parralel and also are they twin or single voice coil. like are there 4 individual terminals on each sub or just two. Your amp should be able to produce enuf power at 2ohms to run those two subs, so i would suggest the problem may be in the wiring.
Also wot guage wire do u use for power and earth?