I play BluesHarp through a Fender Blues Junior with a valve substitution.
A 12AU7 in place of the 12AX7, less feedback and more volume.

I get my gain using a HarpBreak pedal made by LoneWolf.
I blow into a Shure Green Bullet or a Hohner BluesBlaster hi-Z mic.

Check out 'Jozaphine' and 'Walk Away' at:

www.soundcloud.com/the-hubcaps

For interest, Benjamin Craig advised me on the best Amp Modification for
my harp requirements.