The current thinking is that multiple subs are the way to go. I've messed around with subs, equalization, real time analyzers and measurement mics to get an understanding of how my perception relates to measurements. I haven't done it yet myself but I'd buy multiple cheaper subs if I were you. For one, your room dimensions of 13x26 are almost a perfect 1:2 ratio and will almost certainly have bad resonance problems. Something like 4 of the below that can be placed around the room. The individual subs would be doing a lot less work which would increase dynamics and reduce distortion.
YAMAHA YST-SW315 10" 250 Watt Powered Subwoofer Black | Accessories4less
YAMAHA YST-SW315 10" 250 Watt Powered Subwoofer Black | Accessories4less