That's interesting. When I was trying to get more bass out of modded iM716s the flange tips were more bass-lite than the foam overall- and then even the composition of the foam (urethane, PVC and even memory foam) varied the sound. I basically built a number DIY foam tips from foam ear plugs.
All tips sealed well, at least passed the ear suction/attenuation of outside noise and increased microphonics factors. But yeah, I did run into some foamies that were too small for good seal and those all yielded poor bass, as expected.