This issue has come up from time to time. We prefer you directly connect the source signal (if your DAC or preamp has dual outputs) to REL. In the "old days", amps have more distortion; therefore, REL advises using the amp's output to connect to the subwoofer to have the same sound characteristic. This is no longer necessary.
ST-10 has floating ground, we don't recommend connecting REL to the amp's output. But if you want to do that, follow REL's instructions. I think REL has added DC blocking capacitors in their subwoofer so it is safe to do so, but we still don't recommend it.