Bobbsy
Boring Old Git
I wouldn't say get another Behr speaker as a monitor - frankly they are ok as monitors, but the sound can be a little harsh from them as you push them - the SPL is pretty low for a main speaker. But for a small winetasting gig like you describe, they're fine. It probably sounded better to you when you hooked everything up to face you because you didn't have to crank the Behrs up, reducing the overall harshness.
The Marshall is an acoustic voice + guitar amp, so also is fine for using how you're using it.
If you find yourself needing more volume (larger venues/gigs) look at getting some better mains, and using one of the Behrs as your monitor, and get a proper mixer to use instead of the Marshall.
Academic now, based on later posts, but...
I wouldn't normally suggest a B210 as a monitor but, in the is case, it would let a solo performer hear exactly the same tone that the audience is getting. It's also dirt cheap and sounds surprisingly good for the money. A very small amount of HF cut on the built in tone controls (calling it EQ is being too kind) and there's little harshness and a surprisingly clean, loud SPL.
As I say, it's far from the best out there but in this particular situation it might have some advantages for the OP.