Tuning to 13Hz will greatly increase excursion in the more important areas around 16-20Hz, certainly you'll lose quite a bit of headroom for a pedal that doesn't exist (?). 18Hz makes more sense when you actually think about it. Also, that's a big "if". You shouldn't expect a "single 1220s output" (not sure what frequency you mean, but anywhere in its bandwidth) at 13Hz. The porting, excursion, and power requirements go up greatly as you go lower.