[eclein]

I have a bookshelf speaker (JBL-L20T), its 2-way and the specs say it has "12dB/OCT crossover" is that second order? first order? I would appreciate it if someone could explain "XYZ order crossover"- what does that mean? If the crossover is 3000 Hz-12dB/OCT would I be able to just swap out the existing tweeter with another with a suggested 3000Hz crossover. I'm thinking about pulling the tweeters and replacing them with these HiVi planar ones:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=297-400
Would that work or do I really need to rebuild the crossovers, I'm not looking for perfection I just don't want to blow stuff up...

[opnly bafld]

1st order is 6dB/oct, 2nd, 3rd, 4th are multiples of 6.

Since raw driver specifications are so varied crossovers are usually designed for specific applications.

Lin

[*Scotty*]

eclein,here is link to the pdf doc on your speakers.
http://manuals.harman.com/JBL/HOM/Technical%20Sheet/L20t%20ts.pdf
I would be tempted to leave this speaker alone. You would probably be better off with a kit that has been engineered to work right from the beginning rather than to try grafting a different tweeter into an existing design.
Scotty
                         

[S Clark]

What are you hoping to achieve by switching to the HiVi tweeter?  Looking at the schematic for the crossover in the link that Scotty posted, I see that they have at least included bypass caps (at least an effort was made to improve sound).  Depending on what you are trying to accomplish, you may can address some issues by tweaking the tweeter crossover. 
Looking at the specs on that HiVi, I doubt that you would "blow it up" by putting a pair of them in- however, your chances of getting a flat response are not much better than winning the lottery. 

[eclein]

Thanks guys..I got a sale email from PE and the wheels started turning, I have the N2X speakers that are making the rounds and was thinking of trying the planar tweeters in the L20T's to try and create the same type of sound but it would be a crap shoot at best I'm seeing. I'll save up my cash and do it correctly with the proper kit when I can.