I feed back with my spider II all the time. you just gotta crank the gain, get the volume at a pretty decent amount, and throw the volume know on you guitar way up, then face your amp, put the guitar nice and close, and voila!
if you are running through a distortion pedal, get the gain nice and high. and I like my tone to be more towards the treble side of the spectrum, but that's up to you. experiment to your heart's content.
my problem is getting feedback when I don't want it. I was actually going to make a thread about it, asking for advice, but I suppose I can just ask here.
alright, so, we are at band practice, and I'm on a good crunch channel, playing rhythm. when I have a solo coming up, I switch to my lead settings on my OD-20, which is very high gain (though I have my level adjusted so that it's only a little louder then my rhythm channel). and as soon as I take my fingers off the strings, feedback... not the buzzing kind, I'm obviously going to get that... I mean the high pitch squeal thing that you are all talking about. I tried standing about 10 feet away from my amp, with my back facing it, and it still was feedbacking... as soon as I start playing, it's fine, and if I keep my hands on my strings, it's fine... but if I let go, it just wails! I suppose I could always adjust my settings... but I like the tone I found! someone told me that a compressor would help stop feedback, this true?