In theory, it would be great to use the 4x10 speakers in the sessionette as it's own cab without the amp (assuming they will take the Mag 600), however i'm not sure how easy it would be to implement that.
Perhaps a cheap used Behringer BB115 to compliment the BB210 would work well?
Mr.Foxen - Could you describe a bit more what you mean by running the ashdown into the sessionette? Would running the ashdown head into the sessionette amp work? I.e. are you saying it just takes the signal and then lets the sessionette run as loud as i've set it, so it works independantly from the Behringer, which will be taking it's signal and power from the Ashdown?
Cheapest option is the best atm
Basically, you plug the Ashdown head into the behringer cab as one rig, then run a lead from the tuner out of the Ashdown (supposing is same as my ashdown) into the input of the Sessionette. Might need to eq fiddle. Score a big cheap 15 and run the ashdown into that with the treble right down, and turn the bass right down on the sessionette would be ideal way for this.