On attaching the 840 pads -- I've always found it easiest to take the flaps and roll them over the earpads, so that they are inside-out. Then attach them to the baffle, and "unroll" them onto the headphone. Over time, the pads do stretch a bit and get easier.
However, if I were to desire a slight more "sizzle" in my upper frequencies with the Fostex, what's the best approach? Is placing a piece of foil on the back, center square of the Tic-Tac-Toe driver back the best way to go? Or, should I consider something else to apply? At 48, my ears tend to desire a bit more upper frequencies either because of some hearing decay or limitations in the frequencies that I can hear. That's why the Grado - and, then the Magnum has appealed to me. Bear in mind, the Magnum has provided some of the best balanced sound that my ears have heard in a headphone. But, I suspect the mids and lows in the Fostex will still be slightly predominant - in part due to their design and what I can hear even before the modifications start. So, I'm essentially asking the best best approach to "tip" the upper frequencies a bit. Any ideas on the best move to make?
I think based upon all of the work that BMF has done, it seems you want to damp the driver more in order to tip them toward higher frequencies. In his experiments with transpore tape, it was determined that the more layers placed behind the driver, the brighter the overall signature got. Bass performance seems to be determined by the seal of the earpads, as well as volume of air that is moved. Conversely, with more airflow restriction, the higher frequencies seem to come out more.
I would do that sort of secondary tuning after you do the basic modding -- acoustic foam, namely -- to get rid of as much resonance as possible. Once things get cleaner, you may decide there is enough clarity.