I just got done putting a pre-wett/ straight liquid app system on my V-box spreader. I have the two pre-wett tips installed on the chute as the material drops it hits it before it gets to the spinner, I have not tried Magic-O yet, we have only used Liquid Calcium, and we just put it on on Saturday, so we dont really have any good experience yet. I will post some pictures and keep everyone updated.
Our setup sprays the salt at the top of the chute,as it comes off the conveyor chain.Any further down,and it's doesn't do a very good job.If your feeding a lot of salt,it doesn't get it all,but it helps.
I would say the pre-treated salt,works better as your concentration and eveness is much more controlled,as it is all done beforehand.
Prewetting has the same benefits,if you can get the same coverage which is much harder on the fly.Pre-wetting stops the salt from scattering and bouncing when it hits the pavement.Something that pre-treating does not do,unless it is still really wet.
I work for ODOT. All of our single axle dumps have tail gate spreaders with a chute that drops to the spinner. We have two spray tips in the chute that spray the salt as it drops to the spinner. All our tandems have v boxes w/ two spray tips in the chute as well. Pre-wetting helps keep the salt on the road and makes it work alot quicker, especially when useing liquid calcium chloride to wet the salt. ED