Weird tone/bonehead move

Friday night gig at a Legion. I did load in & setup at 4:30, after my day job. 2 x 112 cab, Markbass head... simple. Rolled in 10 min before set time, tuned, checked power, confirmed output. Couple songs in my tone sounded kind of thin but I dismissed it to room shape, recent bass setup, ears stuffed up with cold. Turns out one of the cabs was plugged into "pre-amp out" directly beside the speaker out on the very crowded back side of the head.

Surprised 112 was enough for that room. The rest of the band didn't notice anything different. Doh!
 
My first real rig had a pretty old Stewart power amp that required banana plugs and an unintuitive wiring scheme to bridge it and properly power the cab (Eden 410xlt) and I can't tell you how many times I messed that up and the drummer couldn't hear me. Lesson learned.