OK you asked, Have done teh replacement front and rear but not with J&M.
Try these many have had good results with this model of speaker.

They will fit front and rear.
Some people are about to cringe,,,(oh boy here we go again!!!),,, here is a link to my Multi-Point upgrade and mainteneance thread...
Dig through this thread you might find a bit of usefull information and pictures....
http://www.valkyrieforum.com/bbs/index.php/topic,21439.0.html
I used the DB401's in the rear and now wish I had used the DB501' in the rear since dimensionaly they are Identical although the DB501'as have a little bit wider dynamic range.