Where do you control the volume? Is your DAC, a pre amp too?
One quick fix to your problem is to get a Y cable to split the Pre-out, and feed to both the power amp and Sub. Another way is to introduce a minidsp 2x4, the non HD version between your DAC and Power amp. If getting a DSP seems costly, stick to Y cable. But introducing a dsp filter opens up a wide scope and enables you control and better integrate the mains with sub. I see you've quoted REL's high level idea, but low gain / rca is a proven robust connection to any amp, including subwoofer plate amp.