The Philips SHE3580 (and by extension 3570, 3590) have a vent in front of the driver next to the sound tube (revealed when you remove the eartips). I've always had a hunch that covering these holes would increase bass like on the Monoprice 8320, but since I already find the bass plenty strong enough I never felt the need to try this. Today, I tried covering the hole to see if it would increase isolation as well.
First I tried taping the hole (a fiddly manoevre since the hole is on the edge of a round surface, right next to the sound tube), then I glued it shut with superglue. Neither mod changed isolation, but I found that taping the hole shut gave a moderate bass and subbass boost while gluing it shut gave a BIG boost. I don't have any of the other bass iems to compare but I would wager that gluing the hole shut makes the SHE3580 go at least toe to toe with anything you can throw at them from this thread basswise!
Two DIY guys adviced me to use hot glue instead of superglue, but after buying a glue gun I just couldn't see how it could work on the tiny hole.
I just used jelly-type superglue. Better check that your jelly superglue is actually running like jelly though, before you start gluing. It suddenly started running like water when I started gluing my pair and I was afraid the glue would drip onto the drivers through the hole. Didn't seem to have happened though, both drivers sound fine.