#2
Then buy a bass amp.

You can play a guitar through a bass amp, but the other way around will blow the speakers.
#3
Any bass amp will double as a guitar amp... however you NEVER, NEVER, NEVER play a bass through a guitar amp!

Ever hear of this little thing called a Fender Bassman? One of the greatest guitar amps that just happens to be a bass amp.
#4
and the Marshall JTM45, 1986 (bass spec 1987 50 watt plexi), Marshall superbass (bass spec 1959slp). All sick guitar amps.
#6
Also...

Although I couldn't find much info about it, the Yamaha G100 looks to be a guitar head... I may be wrong about this but everything I found about it said "Yamaha G100 guitar amp" - I wouldn't risk trying it out with a bass and ruining it.