Like Jeff says, you need to be able to attenuate the more sensitive amp. An active crossover is always best. You could try these although I've never used them myself:
RCA line-level attenuator:
http://www.partsexpress.com/webpage.cfm?webpage_id=3&SO=2&&DID=7&CATID=48&ObjectGroup_ID=195&manuid=265
Or maybe a pair of the "Endler Attenuators":
http://mysite.verizon.net/vze4c5pt/id2.html
Dave
RCA line-level attenuator:
http://www.partsexpress.com/webpage.cfm?webpage_id=3&SO=2&&DID=7&CATID=48&ObjectGroup_ID=195&manuid=265
Or maybe a pair of the "Endler Attenuators":
http://mysite.verizon.net/vze4c5pt/id2.html
Dave