dispersion patterns, baffle step effect, cabinet resonances, crossover design…
A mate has just had some PA speakers and matching sub designed by the guy who used to run Spendor – passive and run by Crown amps so making a very portable and flexible arrangement – and the dispersion to the side is very impressive – you can stand right out to the side of the speaker with virtually no change in the sound.
The cone and dust cap profile have a lot to do with this – he had to have a custom run of the drivers made.
The look a bit like these that he has done for the Royal Opera House, but narrower :
http://www.hifiplus.com/articles/a-night-at-the-opera/
I have heard many of his speaker designs and they always have an exceptionally seductive midrange – whereas other accurate designs, for example PMC FB1+, are accurate but sterile/unrealistic compared.
I have some of his external crossovers for the LS3/5a and the difference to a normal LS3/5a is unbelievable – way more transparent and tons less THD. Same drive units and cabinet though…