I'm sorry to revive this old thread, but after reading this I later came across some other discussions that have me thinking. Namely, while researching the Meridian Explorer I read about the first version's "problem" with high output impedance; first 50 ohms then 5 ohms after being "fixed". Then, some time later, I came across a post talking about how Grado's do well with amps with higher output impedances, which is why they love tube amps so much. The poster said it had something to do with Grado's being high dampening cans. Apparently all of these factors lead to the high end of the Grado's being tamed in a positive manner.
So, my mind flashed back to this thread and how Deebrewski was using the original Explorer 1.0. Naturally, I'm wondering now if that high output impedance is the reason for the Exploer sounding so good with the 325e. I don't know if Deebrewski's was the early 50 ohm version or the later 5 ohm one, but they are both still significantly higher than the Explorer 2.0 which measures at just 0.45 ohms. Makes me wonder if the new Explorer 2.0 would not sound as good with Grado's.
I don't suppose there's anyone out there that's heard their pair of Grado's through both the Explorer and Explorer 2.0 and can answer the question.
No experience with the Explorer itself, but to the more technical part of your post, I'd have to disagree with whatever you were reading. And here's why:
Grado's headphones, in general, are fairly non-reactive (which means their impedance is pretty consistent wrt frequency), and fairly easy to drive (e.g. low overall impedance + high sensitivity), and as a result they're pretty un-picky when it comes to amplification (as in, their frequency response isn't going to change significantly with respect to source impedance, and they don't need much from whatever amplifier they're plugged into). So high output impedance is just going to make things less efficient (you're going to be dissipating power across output resistors or output devices or whatever (however you're getting "high output impedance")) with them, but that isn't generally a problem (because they don't need much power to begin with).
"Electrical damping" or "damping factor" are largely machinations of marketing run amok (going back to the 1970s, where it should've stayed, and largely has outside of the land of headphones, where marketeers have a whole new generation of customers they can try and hawk their wares on).
IME the bigger "gotchas" with Grado cans (like other cans with similar electrical characteristics) are amplifiers that have low noise, low distortion, and good channel tracking (especially at low levels) to ensure everything sounds "centered" and isn't sullied by noise/distortion.
Also, tube amplifiers don't have to be explicitly high output impedance (and shouldn't be broadly characterized as such) - OTLs tend towards somewhat higher output impedance, but there are other kinds of amplifier topologies that use tubes, which may not have that characteristically high output impedance (and furthermore, there are SS amps that have high and low output impedance, depending again on the designer's choices - the IEC specification calls for headphone outputs at 120R, but this is by far not universally followed). From personal listening experience, I honestly don't notice much (if any) difference between IEC-spec 120R outputs, higher-than-IEC outputs (at 300-500R), or very low Z-out - as would be expected from easily driven, non-reactive cans.
