Feedback. Got any tips for me?

[BurbsToo]

So I was just messing around today and decided to try feedback. I just play for fun, so I only have a marshall practice amp and I live in a dorm (and my damn RD banned instruments because she's a conservative cock-shiner) so I never get loud enough to get accidental feedback. But today I leaned by bass, a virgin (straight hardwire from pups to output) squier P, against my amp and got some feedback so I decided to play around with it. I'm completely self-taught so my knowledge is a little limited. So this what I figured out on my own. When I put my amp to 'modern' mode (dirty) the E and especially D string resonated (btw, D resonates a bit now matter how I play in modern mode, so I think that might have something to do with D dominating feedback.). And when I put my amp in 'classic' mode A dominates feedback. I also noticed that 5ths of those notes feedback well too. I don't really know why this is because all notes should resonate equally well. I know that gain and distortion increase feedback. That's all I was able to figure out in the 10 minutes before class. I'm really interested in feedback on a bass because I think it's much more appealing to the ear than on a guitar. Can I get some tips on how to incorporate this technique?