assistance with box for a hdc315

[Irocthestreetz]

I currently have a pretty well made box for my hdc315 at 4.5 cubes tuned to 34hz but I believe on a couple songs it starts unloading and on anything but hip hop it sounds very sloppy and I'm told this is because I have it in to large of a box for the power I'm giving it (amp is a soundqubed 2200.2 at 1ohm) I'm going have a new box made that's tuned a little lower how much smaller should the box be to keep output but to gain in sound quality? also what style of reinforcments should I ask for? my guy uses 3/4th mdf with 45s in the corners and I want to get everything I can out of this sub