[SteveCallas]

The Plus/2 has roughly a 36db/octave rolloff ground plane, you're showing roughly 28db/octave, so about 8db/octave gain. That's pretty good. Room Mode Calculator shows that gain should be starting ~28hz, and that's almost fully realized with the Plus/2. That would work nicely with a very low tune, I'm thinking 13hz - Steve and Ryan's are both tuned ~16.2hz. Can you afford to make it a bit longer, or is 72" it? 570 liters would mate very well and allow you to get away with an 8" port that is ~34" long.