The manual for the B600H quotes that it is rated for 450W into 4 ohms, 600W into 2 ohms. So just hook up both cabs to it.
Not sure what you're getting at with the "sort of preamp" part, but you won't gain anything from using the 160W amp unless you really like the preamp it has a lot more than the one in the B600H. In that case you can plug the preamp out into the effects return of the B600H, both cabs connected to the B600H, no cab connected to the 160W. So the signal goes: Bass into 160W preamp into B600H effects return into both 810's.
Flatwould Flatwound club member #506
My fEARful build:talkbass.com/wiki/index.php/Oobly
Last edited by Oobly : 03-15-2013 at 04:45 PM.