To avoid the problem of unbalancing at the amp end I always carry with my rig 3 or 4 giant jack plug adapters - they reduce and XLR to a sack plug, but there is a little transformer in there ro combine the signals. They are switched to offer impedance matching too. The only real problem with them is that if the jack socket you plug them into is not in perfect order the weight of the adapter wiggles the plug out of line and you get an unreliable contact. This can be solved by carefully wedging 3 beer mats underneath and fixing them together with Gaffa tape (a truly professional solution)!