These are great. I have one and use it on both guitar and bass cabs. I also use it in my distance kick drum mic. I dont know how this compares to crates or bricks however, I just know it works for me.
I have personally liked my results better off the floor. I've used everything from bricks to chairs. I also like carpets better, but it does depend on the sound. If I'm recording reggae, I prefer tile.
Sorry to be this guy but this is from Mark, not Jason
I know Mark and Jason work as a team, and Jason is an awesome producer/engineer/mixer... But that really annoy me when people credit Jason when Mark done the job...