My guess is that "richer" indicates a fuller midrange. I doubt that a sub will improve on that "richness" per se, but you'll probably notice a stronger low end that has a bit more punch.
This could contribute to how you hear the midrange, but not necessarily. Could also be the content he or you put through your systems. Better quality recordings, or better mixed tracks could be a contributing factors as well.
Best way to figure it out is to bring over yours or his speakers and do an A/B same room, same placement. Play the exact same content and maybe bring your receiver so that you can do some quick switching. It's a lot of effort, but it's something that I would do before I go out and try to fix a problem I haven't necessarily identified yet.
You may learn a few things about both sets of speakers that weren't apparent before. You may find that you don't like the B&Ws as much as you thought, or you may find that the differences you heard were in your head. Who's to know unless you try
