1. Would be just guessing.
2 .Understand and mapping a rooms bass modes and nulls is easily accomplished using the previously mentioned crawl method. Whether using one sub or more the crawl is a terrific first step that only needs to be once.
3. Value, yes. This will most likely be a function of trial and error unless you have equalization and room optimization control such as described in jrpnde's Soundoctor paper. David Hall's Digital Drive and DD Plus designs offered most of those parameters using automatic / manual software and a test tone CD to plot both the mains and the subs. The point is, even mismatched subs with basic controls when given a processed signal can add sonic value to a low frequency system. Keep your subs.
4. Put on some Prince, turn it up and dance with the wife. All the best.