Can you borrow a sub to see if it helps? If you went directly to Khorns, the bass improvement should be there, but, you might still feel the need for a sub; some do and others don't.
I've never seen or heard it discussed here, but could the djk La Scala bass bin mod (see thread http://forums.klipsch.com/forums/p/76770/757339.aspx#757339 and threads referenced therein) be practically applied to a Belle? I know something similar could be done, but would it work as well, would be be easily reversible, etc.?
IMO, Belles are too pretty to permanently alter the appearance to gain a little more bass.