T nuts are probably the best. Few problems I have ran into with them though. Make sure you glue them bitches when you install them. Nothing like going to tighten like the second to last bolt and the damn t nut falls into the box. Then you have to take them all out and start over. Another issue I have had, Even when you screw them, In MDF it is easy to over tighten them and just plain tear the MDF. Also you better make damn well sure that speaker doesn't move when you are drilling all the holes, just one more thing to fuck your day up when you tighten like half the bolts just to find out some of your holes are not lining up right. Just a few things I have encountered. Also if you dont cut the hole perfect, like if you cut the sub hole a tad to big, the t nuts can stick out into the cutout and really provide next to nothing for strength.
But if you have none of these problems they should work great! LOL. Another option is go to your local hardware store and get you a box of #12 or 14 hex head screws, make sure you pre drill you holes.