I have two valve amps that have 8 and 16 ohm output options, another that can do 4,8 and 16ohms and a solid state head that has max. output into 4ohms. I (will soon) have two 1x15 cabs that are 8 ohms each. For the 8/16ohm valve amps to run both cabs I'll need to run them in series somehow to get 16ohms. Two options spring to mind: 1. I've heard some talk here of a 'series box'. Are they commercially available? (I imagine that I could make one) 2. The new cab I'm building... in addition to the regular input (will be XLR like my existing cab) could I sensibly have a 1/4" switched jack socket in the circuit in series to run to my existing cab when plugged into this socket? Any other suggestions gratefully accepted.