Multiple channel amps are for speakers, and mono amps are for subs. A 4 channel amp will power 4 speakers.
I got my hiphonics amp for a little more than $100 on sale. You should install the system yourself, its a really simple process and it will save you money. Watch a few youtube videos and...
App.
I used it when I was cycling. It maps out your runs/rides, tells you elevation change, speeds in different areas compared to other people, etc.
Really useful
I was driving on some old backroad a few weeks ago. I'm doing 55 in a 45 and a truck starts tailgating me. A deer runs out into my lane so I swerved into the other lane to avoid the guy behind me rear ending me. (There werent any other people on the road). The deer ran away, and I let truck dude...
http://m.neverlightagain.com/category/38
The chrome one was actually the first vape I bought. It lasted months until I started building. Variable voltage, decent sized tank, and fits well in the pocket.