putting castors on subwoofer

1) Take driver out/anything else that might be blocking your view like wadding or cabling

2) Choose the locations and orientations of your castors so as not to hit anything you shouldn't 

3) Select suitable bolts (hex head are good, socket/allen ok too) and T-nuts. Also use a lockwasher on outside between castor and bolt head. Bolts must be at least long enough to engage all of the thread in the T-nut and not be too much longer.

4) Hold castor in place and spot through the mount holes with a Brad Point drill bit of matching size.

5) Drill clearance holes for the bolts at marked spots. Drill them straight! Drill them slowly! When you get close to finished (and the point has emerged on the inside) stop and finish them off from inside (space allowing) this prevents tear out. If there's not enough space just drill very slowly with little pressure. These holes should be an EASY fit for the bolt I.E. the bolt should drop into the hole and NOT bite/thread into the wood, only into the T-nut. Generally this hole will be same diameter as T-nut's cylinder part.

6) fit your T-nuts using a big washer and with clean lubed threads on the bolts.

7) finally fit your castors, Bolt them on as tight as you can, consider using threadlocker or (my favorite) just grease the threads and use a serrated lockwasher. 

8) run it up, play low level infrasub into the box and listen for air leaks around your castors, if any then fix them.

9) Done! 

If you follow all that i can't see it effecting the resale value badly or going wrong really?