Yes, I have seen his mod. Essentially, the rubber insert is similar to taping the front nozzle. Both are dampening the nozzle to reduce higher frequencies.
Replacing back filter is somewhat similar to closing the back vent on the shell (i use micropore tape). Both mod reduces the mid bass and lower mid. A member on another forum explains that by choking the driver, we can tightening is transient response as well.
Opening the front vent like SB would further reduce the midbass. I haven’t tried it.
Hey, you are that modder on YouTube! You inspired me to start modding. We discussed in comments a couple times about the impact of 10k dip on 3d soundstage.
Anyhow, opening back filter certainly increase midbass. Dampening it (adding filter that you did or taping off the vent like I did) reduces the midbass while not impacting the subbass that much. With some IEMs, opening the back vent also increase the soundstage width and resolution, at the cost of bass bleed.
A few things!
I think the rubber part isn't fully similar to a front vent, it messes with the acoustics much more.
Other than that, replacing the back filter with something else really depends on what you replace it with haha, it can go both ways.
Opening the back vent will ALWAYS increase soundstage, in all directions
On the other hand resolution really depends, if by closing the vent you get faster transients then by opening it they slow down, meaning you may lose resolution.
On top of that it depends on the driver, and the way it's FR interact with having the back opened.
Bass bleed is the same, depending on the driver, fast drivers like planars will handle the midbass boost just fine
IMO, kudos to anyone that can take a close back iem, mod it and make it work, but usually if you go for an open back iem, from the get go you'll most likely play around with different positioning and drivers.
Could you imagine someone modding an lcd i4 and making it a closed back iem? the treble will probably become hell on earth and bass will be anemic, however if audeze would make a closed back design they'd probably opt for a completely different driver
just food for thought!