I'd like bring out vocals and guitars more on my MK3's. Would mods directly on the backside of the driver help with this? I'm not too sure what those mods actually do to the sound signature. Are they easily reversible? I'm afraid damaging the stock white dampening on the driver if I stick felt on top of it and then try to remove it if I don't like how it sounds.
Read page 1 in this thread for numerous ideas to try, a description of the typical results from various modifications, and tuning tips.
Note: these mods were applied to T50RP mkII, not mkIII. Trial and error testing of these methods with mkIII will be necessary given the changes Fostex made to mkIII:
1 of 9 rear grids has the white paper cut out
controlled venting of the baffles
venting on the ear side of the baffles via side slots where the lip of the pads are installed
Do not apply tape directly on the white dampening paper. If you do, the glue will eventually liquify and foul the white paper's damping properties. Instead, temporarily secure the felt with strips of tape around the perimeter forming a "picture frame." This is shown on Page 1 and this method is totally reversible. Once you settle on a modification scheme, you can permanently secure the felt over the back of the driver with a bead of hot glue all the way around the edges of the felt and driver frame.
Stiff felt seems to work better than soft felt. If you don't have stiff felt, try a double layer of soft felt.
I don't think plasticine, Newplast, or Dynamat are worth the expense and effort and they are difficult to remove.
The most bang for the buck sound quality modifications are:
pads
modifying the 4 cup vents
adding various dampening materials including cotton, felt, fiberglass, wool, and/or rockwool