i dont know jack about sound gear, but as for the feedback, i get quite a bit in my garage. make sure you arent near flourescent lights if possible, or any other electrical devices. Try putting the amp in every position and angle possible and move yourself and point the guitar different directions. Its just trial and error. Sometimes, unnoticable radio frequencies can wreak havoc. The coolest thing I've ever seen was a guy who built a metal cage and put it over his amp, surrounding it. The effect is like a Farraday cage, it blocks out electrical and radio signals pretty well.