I think it is a driver limitation.

All orthos seem to have this limitation. I'm sure there is a way to boost that range but do you really want to?

i see. well, i was more of being curious about it. if i want to boost it? maybe. maybe not, but, i would try to if i could just to see what changes in the sound it makes just out of curiosity. it's mostly just being curious.

My vents are fully open as well (with stock felt in place), and one thing I noticed is that there isn't much difference when I close them from the outside. I can tape them up and then rip them off and there doesn't seem to be a difference (none that I can detect, at least). I think once you damp the inside past a certain point, the vents don't matter so much.

Would be interested to hear/see what you did RD.

Also, how does this Fostex stack up against your other orthos?

i notice that as well and experimented with the vents. i ripped the felt off the ports soon i opened them up cause they seem unnecessary in the first place i think. it reminds of the akgs felt blocking the openings inside the cups. it's just there to minimize sound leakage if you use for recording around a mic so the mic don't pick up the sound bleeding from the headphones.

over the weekend i will experiment some more with mine but i been letting my friend use the past couple weeks. i might recable them as well. i heard the pcb is pretty sensitive so i know to be careful. i wish though headphone drivers were made with terminals like speaker drivers where you just can pop the terminals off the driver when you need to replace or upgrade the driver. make re-cabling much easier. i might do that with the fostex drivers maybe but probably not maybe since i really not much a cable person and don't bother with cable testing really.

EQ =/= T50rp modding ;-) 

Covering the vents does have a substantial effect it's a great final touch tuning tool if you wind up needing more treble/less bass.  People in the T50rp thread say closing off vents gives you tighter bass, but that's only if you have too much, it's not a general rule.  IME it really just lowers bass output and increases treble/upper mids

You do not want to remove the damping over the vent.  This driver needs pressure behind it to operate optimally.  I've played around with opening up orthos a lot, some can operate like that, and some need pressure.  The T50rp needs it more than any other driver I've worked with. 

i will try that with the vents. i never had issue with the bass(except the obvious imbalance between the left and right driver under 50hz). first layer was just some fiberglass about 1 1/2'' thick in the cups over the port. something like fiberglass or mineral wool will give the biggest absorption benefits when it comes to the bass and sub-bass(that's why fiberglass is used for speaker boxes and room acoustics). i think something like polyfill/fiberfill due to it's properties of slowing down mid and higher frequencies can help with the quicker/shorter sound waves in the cups. that's all i used in mine was layering of fiberglass and polyfill(like i do with speaker boxes). i found putting foam in front of the driver as well(i had the 240 foam disc underneath the shure pads) will make a nice affects on the bass and treble response while maintaining clarity. i tried coffee filters and different layering of speaker cloths but coffee filters no matter on the layering and sequence came out sounding weird. i'm curious how a very thin of silk will affect it as well. it's pain getting the sub-bass correct but after getting the sub-bass correct everything else fellows in order from playing around with it.

i will try to play with the vents like you suggest and see what i get before i attempt of opening up the cups again. i wonder if cutting off those shock absorbers too will do anything cause they just seem to do nothing but take up space.