Any advice would be welcome please. I have a 1960s Selmer Treble & Bass 50 Valve (Tube) amp head. It has two speaker outputs, 1 @ 7.5 ohm and 1 @ 15 ohms. At the present time I only use it in the house as a practice amp driving an 8 ohm cabinet at very low volumes which works fine. I am wondering about taking it out on gigs just as a back-up in case my main amp should fail but am not sure what problems there may be. I have in mind that I would drive my 2 x 8 ohm cabs connected through in series box to the 15 ohm output. My main basses are both active, a Gibson LP 5 string and a Bolin NS fretless 5 that has an 18v system. Can anyone please advise/comment on what problems if any I might encounter or even whether it is a good idea at all?