Usually when I drill MDF, I just eye up the screw shaft with different drill bits. Most of the time I use standard sheetrock screws, both #6 and #8... I'm not too picky. I select a drill bit that is just slightly smaller than the diameter of the shaft. Then I use either a countersinking bit or a 3/8" or so bit for the head. Go slow if you are using a regular drill bit for the countersink. It can easily grab and end up being a much bigger hole than you intended.

keep in mind that I usually don't use MDF for sub boxes. It'* more for woodworking jigs and whatnot but the principle is the same.

alright you're holding out, what do you usually use for sub boxes.

LOL. Sorry if that was unclear. It should read: although I haven't/don't build sub boxes, I use MDF for making woodworking jigs and such.

MDF is probably your best choice for subs.