I am planning a blowthrough in my gmc truck and I have a question... What shoould I use to deaden the bed? Do I just use the damplifier pro or should I use something similar to spectrum to spray in the bed?

I'd probably stick with Spectrum for the bed. It's going to be cheaper, and more than likely satisfy you. It's a lighter option as well. Make sure and spray it on thick.

Vibration mat (damplifier) is a permanent solution, and if you ever decide to take the equipment in the bed, you'll have to cover the mat in some way to protect from the weather and using the bed itself. A Spectrum coating is a very tough texture and will almost double as having your bed rino lined.

That was gonna be my next question. Would it be ok to rhino line over the spectrum?

I don't see why not, but I'm not the expert. Why not just call SecondSkin and talk with them? They are very helpful.

rhino line will be fine over spectrum and will also do a lot of the same things that spectrum will