Don't get me wrong, the SA5000 are a fantastic headphone. But they feel like they try too hard to do what the F1 just have naturally. Both of them very much fit into that weird line of Sony headphones that you question how they came into being, and simultaneously why they were so abruptly removed from being.
IMHO it isn't quite "thin" though - they still sound right when connected to something like the Recon's integrated amplifier, and they don't really change in tonal character that much. It's more of a bass texture thing, if I had to put a finger on it, but I'm baffled by it because Sony specifically states that this model has a Zobel to *prevent* that very issue from occurring (unfortunately nobody has ever measured them, that I'm aware of, to see what their impedance looks like).
I know that amplifiers that can deliver a lot of power to their headphone outputs, like my Yamaha stereo receiver (it's 100wpc into speaker taps, and it's that tapped through a pair of resistors into the headphone jack), will get along very nicely with them. But I've also encountered some high current opamp designs that do the same thing. They're an odd-duck - on one hand they really don't care about amplifier matching, but on the other hand they do. *shrug*
On a more technical note:
If you're going to build an amp, or experiment with buying amps, make sure whatever design you end up with is absolutely known to be stable into 12 ohms (a lot of designs are not, especially tubed designs - I'd actually even suggest double checking with amp manufacturers before buying a finished product - that 12R is a real killer). I know that most amps on the JR4556 are considered stable there, and I know that the TI TPA6120 is right on the edge of supporting them (I think 8R is it's absolute DNE, but TI suggests 16R and higher - they work very well with TPA6120 devices though). This isn't even really a matter of sensitivity/output, it's just that some amplifiers really can't deal with that low Z safely.
And yeah, I would absolutely say (to anyone) if you have F1s, and like them, hold on to them. They're evidently gone for good now, and they're not something I see on the used market that often either. Truly a once-in-a-lifetime pair of cans.