If the sound output is weak and you need volume control after the HU, then it seems to me that you would need TWO CL-RLCs. One as a pre amp after the soundcard, and another before the amp. The pre-amp part is a set and forget thing, AFAIK pre amp levels are to remain 50% and up, set once, unless you switch sources.
So if you can't use the HU and need a volume control, then yes, use another CL-RLC. I can't see why diff. balanced control in the cl-rlc could cause any problems. I use it as a pre-amp, never touched again; volume control is done by the HU - > amp.