I've not been into vintage Pioneer before, but I just now got a pair that isn't talked about much, the SE-90D:
Not a very ritzy picture, but it's past midnight and I don't have a lot of desk space to shoot either, so.
Some specs and better pics
here, and also of what appears to be the SE-90D II
here – I'm not sure what the difference is driver-wise between those two versions. My pair is missing the cool golden rings on the backside.
There's some extra info on the related models and the driver design
here if you scroll down the page.
What got my attention was the listed frequency range: 4-50000 Hz. I don't think I've seen a range that silly for any other pair of mid 80s dynamic phones...
The sound wasn't terribly impressive with the original pads, though they're dead flat and I'm not sure if that's how they were originally. With a pair of pads off an HOK 80 and some blu-tack stuffed inside the cups for extra mass (these are almost closed phones), the sound is a lot better, and actually pretty decent for such a light-weight pair of old phones. The treble's a bit... strange, though.
I can't properly measure on-ear phones, but from the graph I did measure (with the HOK 80 pads), the SE-90D seem reasonably neutral, only with a dip at 2.6 kHz or so that makes the sound a slight bit recessed. They definitely don't go down to 4 Hz, though, not with these pads anyway.