I am copying this from the response to your facebook message.:
As far as baffles go, a flat baffle will have the least issues with resonance. You can get a little more output from H or U shaped baffles, but you also add in additional reflections. As long as these reflections are not in the operating bandwidth though, it would not be an issue. With 4X Dipole18 per side though, you should have plenty of output capability. You should have much more output even down at 20hz than the RD75 is capable of over it’s intended range. I would not expect that you need to go with an H baffle to gain more output.