Do aftermerket crossovers improve sound?

Ya, you could go active but that would require multiple amps or a multi-channel amp (Accuphase and Ayre come to mind) and the 802's are not an easy load.

http://home.netvigator.com/~goneill/bw.htm

Too late...

http://cgi.ebay.com.au/Accuphase-F5-crossover-350%2F2.5KHz-for-JBL-B&W-801-802-_W0QQitemZ220557442619QQ

Shadorne amazes me how much folks pay for cheap transducers and poor measuring crossover parts. As long as it has the right brand name or had a + review fancy veneered of coarse. I see many 15k loudspeakers with $50 tweeters. In our KCS 17K model we offer fostex amk2 a $1550 tweeter set. Our capacitors cost more than most all parts used in out competitors loudspeakers. But your right many folks do not buy on quality of design and parts just on validation. There loss.

Give Bill Legall of Millersound a call to see what he can do for you;he is one of the most regarded people in the industry for speaker repair and crossover redesign.

The simple and straight forward answer is yes. Speaker manufacturers neglect the internals of speakers when they are trying to keep costs down. They know the 90% of people aren't even going to look inside.

Even if you just replaced the critical parts on the xover, it would be a great improvement. When I upgrade xovers, I focus on the high and mid parts, a lot less to be gained from the low portion of the circuit. Furthermore, I typically just swap out for like values, and only on ocassion, do I bypass caps.

Bottom line, it's right up there with the best money you'll spend on audio.