It's the workings of a number of things ;-) The craft foam is great for resonances in the cup back, but it isn't enough. I'll probably post my mod soonish, it's a unique mod to LFF's, we didn't share notes, but we do seem to share the same general goal and preferences.
LFF's are completely sealed and totally closed back, mine have the vent fully open (but with stock felt in place). So sealing/not sealing isn't a necessity, it depends on how you're tuning them. Closed has it's advantages for isolation, but the more open back an ortho is the bigger your soundstage will be in general. Of course there's other things that effect that as well.
