Hello Musiclover,
"Also, on a Ballpark percentage scale... if the first sub adds an imaginary 100% to sound of a system - more warmth, better voice, realism, etc. how much does the second do... another 20... 50... 70%?! Just to get a rough indication."
In my opinion adding a second sub is about space and equal pressurization of my room. It's very hard to quantify the percentage of change, but I would say 10% or less as a rough estimate. As with everything else audio related, despite the "small" percentage regarding improvements, adding the second sub, once you "hear" the difference it's hard to ignore. I submit this hobby is usually about incremental system improvements that are relatively expensive.