Yea, the T50rp really does improve with very simple mods. Mine are modded pretty closely to yours actually, minus being balanced. I also used a piece of stiffened craft felt right behind the driver and I am using the Alpha pads. Another thing that can make a huge difference is tuning the bass venting. Covering the outside 4 semi-open slots with masking tape, then trying vents of varying sizes will really dial in the bass quantity and quality. I'd try sealing it completely, then start with a single hole the size of a needle and keep making the hole bigger until you get the desired effect. Right now I am using a single hole in each side the size of the tip of a ballpoint pen. Good bass speed with just the right amount of bass quantity and impact. Of course every single mod will vary depending on each and every other single mod, the possibilities are endless.
I recently picked up a new set of T50RP headphones and tried them with the stock and Shure 840 ear pads. I was hoping the ear pads would be enough of an improvement I'd be happy with the sound. I wasn't. I like the monitor-like neutrality of my AKG K240 Studio phones but I just did not like the T50RPs at all so I decided to do some of the standard mods. I was pretty skeptical that simple mods would improve the sound quality of the T50RP.
I added a male 4-pin Mini XLR socket and wired the drivers for balanced operation. I built a cable using Canare Star Quad cable with a female 4-pin Mini XLR on one end and a male 4-pin full size XLR on the other and made an XLR to 3.5mm plug adapter for single ended ops. I filled the headphone baffles with generic modeling clay, lined the cups with self-adhesive felt (two layers where everyone puts the sound deadening material), covered the driver's central square with magazine paper held in place with one full length of spore tape and added 6 cotton balls per side.
All I can say is wow. It's pretty amazing what even my lazy man mods do to these headphones--and I'm using them with a single ended adapter cable so the sound improvement isn't from switching to balanced output (but a Schiit Mjolinir is on the way). Right now I'm using an Aune T1 DAC/AMP to drive the phones. I've been going back and forth between the T50RPs and the K240 Studios and I'm shocked to find I prefer the modded T50RPs.
Now to mod the K240s for balanced operation and make an XLR to banana plug adapter so I can try the headphones with my receivers' speaker outputs.