This is just my personal opinion and not a personal offence in any way: Trying to adjust the sub and mains volume manually, re-matching it every time you intend to change the volume, is such an incredibly bad idea. It's not only the mother of inconvenience, it also makes high fidelity impossible since it totally goes against the aim of smoothly blending mains and sub.
Technically, it might be doable, though. But I will vote against it.