Why would you use Stax as portable cans?
Given their rarity and how fragile they tend to be, I would HIGHLY advise against this. The SRD-6 and SRD-X both perform the same task. Both provide a 230V potential to the associated ear speakers. The SRD-X has a bit more capacitance due to the internal battery, but, this has no effect on sound quality.
The SR-5 "gold" is nothing more than the SR-5N. The specifications on the SR-5 and SR-5N are identical. Frequency response, bias voltage, max SPL, all the same. If you can get the SR-5 cheaper, then by all means, do so.
Sorry I forgot to reply to your last PM. I do that, sometimes.