Control volume at speaker cable level for A/B??

Hello,

I have a speaker-level switchbox, and I want to do some A/B comparisons between speakers. The volume control on my switchbox is to coarse to effectively match the volume between the speakers.

Is there a way that I can attach a volume control device to one of the pairs of speaker cable so that I can match the volume of the 2 pairs of speakers precisely?

Is there an off the shelf product that will let me do this?

Thanks in advance!!!