Re: Guitar through 18 inch speakers. .
I swear by a 15 inch speaker for a smoother bass response. They are great for downtuning.
I have a 1966 Super Reverb that has a D 130F instead of the stock 4 x 10s and it is has MONSTER tone. I used to use Marshall half stacks but have stuck to 15 inch speakers for the last 17 years. I even monitor my Line 6 through a Polytone Mini Brute III which has a single 15 inch speaker.
For proper speaker cabinet emulation, I like the extended range of a 15 inch speaker to faithfully recreate the sound of nearly any cabinet.
An 18 inch might have a bit too much high frequency roll off depending on which one you buy, but it's worth a shot IMO.