Duffinator

One thing I would mention is that to address your complaints//observations about your current system you would be well advised ( IMHO ) to lower your crossover point to around 700 or 800 hz.

That will of course mean a different horn.

I personally don't think all the effort of designing and making new cabinets will be worthwhile without that change of horn ( and lower crossover point ).

That will help with imaging and soundstage ( with the right horn ) and relieve some of the response area that makes this le14h family sound tubby and slow when pushed too high in its response.

There aren't many choices ( actually ) that I could recommend but one ( for the low low money invested ) would be to try out a pair of the Eminence H14EA ( currently on sale at Eminence ) .
- This horn is a BiRadial by all accounts ( as much as any of JBL's are ) .

Your driver is of course a 1.5" exit ( not 1.4" ) so you might want to instead consider the Eminence H2EA Horn with a suitable 1.5"-2" throat adapter.

Just another perspective ( and I suppose that since your current setup is over a decade old, it must be quite acceptable as is ).