It can be any number of drivers. You just need to ensure the mass moving one way is the same as the mass moving the other way..Does a dual opposed design count as mass cancelling, or is it something more complex like the 7201?
I did the old B4+ way back in 2003(?). Pretty much the s7201, smaller, and turned on its side.
But duals like the submersive and s3010 mass cancel too. I think one of the first I say for those were the old mirage bps series.
Power Sound Audio