Go to the hardware store and buy some 1/4" x 20 bolts and nuts. Grind the heads of the bolts to a point. Be sure to wear your safety goggles. Screw the flat end into the sub and lock in place with the nuts. You can vary the taper of the point or try bolts made of different materials to see if they sound different.
If you don't have a grinder, you can buy one for the $50-60 you were willing to spend on spikes and then you will have a grinder for future projects. Besides, you now have an excellent excuse for a trip to the hardware store.
If you don't have a grinder, you can buy one for the $50-60 you were willing to spend on spikes and then you will have a grinder for future projects. Besides, you now have an excellent excuse for a trip to the hardware store.