Well it all depends on the choice of transistors really...Joe's FF uses Russian NOS germaniums which are rather dark sounding, with a little less gain than the silicon FFs (although more gain than you'd expect from a 'normal' germanium transistor FF)...whereas EJ is known to prefer the silicon FFs, which are a little brighter, have more gain and are more controllable (not affected by temperature changes etc, like germaniums are). So I expect EJ's signature FF will be a silicon model, not germanium like Joe's. The only other differences between FFs I guess are parts like the pots and jacks etc, which don't have a massive effect on tone...those parts are more about smooth tapers (in the case of the pots) and reliability.
How similar the EJ FF will be to the Hendrix FF that Dunlop makes (which is also silicon transistor) will all be down to the trannies themselves really...BC108s? BC109s? A mixture of the two? (I had an old Crest Audio 80s reissue FF with a 108 and a 109, it was friggin' awesome)....all shall be revealed in due course I'm sure.
Hope that helps....(please excuse the ramblings of a Fuzz Face nerd
)
Peace
James
'What's the use in gettin' sober......when you're gonna get drunk again' - Louis Jordan
Guitars - Gibson VOS R8 LP, Custom built Telecaster w/Bare Knuckle and Duncan pickups, 60's/70's Kay Tulip (for slide)
Amps n ting -1966 Fender Deluxe Reverb, 1961 Gibson GA30 Invader, Fredric Effects Golden Eagle, Wampler Faux Analog Echo