Ahh now I understand what you are getting at. The cap will certainly do no harm and I suspect that Stax added them when they were having issues with ripple. I know Justin also added one in the amp chassis in the BHSE, probably to make up for the long wire from the bias supply and the close proximity to the filaments on the way.
All of the SRD-7's have a series resistor (I have the schematics for most of them) but it's been too long since I opened up the small Stax amps to be sure. The 313, 323, 717, 007t, T1, T1S all have them.