Why don't you just use both your existing heads, one driving your 210 and a line out to the power section in of the other to drive the 115? Or vice versa. Twiddle the gains to get the desired balance. No impedance matching problems. Done.
Now there's an idea. Ideally I would be looking to shed one of them, but I have listed them in the past and haven't had much luck. I'll have to keep that in mind down the road.