Willie-O,
I'm also using the Peavey 112 mainly for mandolin, sometimes guitar.
With a magnetic p/u mandolin, I have no feedback problems even at high volume.

With the Rigel which I believe amplifies vibrations from the body, feedback was a problem but I never got it sorted out. Even bought the Baggs Para-Acoustic DI preamp which also has a notch filter but wasn't happy with the results.

Now I'm using a Godin A8 (under the saddle p/u). I crank the knob back to -30, hit the G strings and adjust the slider until the hum is minimised. This has worked on gigs so far but, judging from above posts, I need to get more scentific.

It seems too that many of these devices work better in guitar range than mandolin.

BTW, you mentioned that you used a mic at your last gig and had feedback problems. The Ecoustic's 2nd channel does have inlets for guitar jack or microphone. But, as far as I know, the notch filter control works only for the 1st channel which has no mic inlet.