There is some confusion about the parafeed capacitor value for the SEX amp, because of the various plate choke and output transformers that have been used over the years. Here I'll try to summarize.
First, the voltage rating should be a minimum of 600v. We've never experienced any problems with 600v caps. I recommend a minimum 85 degree C temperature rating; some people have used lower ratings without problems but I still can't recommend doing so.
My rule for choosing parafeed caps is 2*L/(R*R) where L is the plate choke inductance in henries, and R is the output transformer nominal primary impedance in kOhms; the result is in microFarads, uF. Usually a range from half to twice that value will give pretty good results, and I've posted periodically about some considerations that might make one lean towards one end or the other. It is something of a compromise between small-signal bandwidth and power bandwidth.
A much smaller cap will reduce the bass extension. A much larger cap won't hurt anything; it is always at least equal to a series feed transformer with the plate choke's inductance.
Here are some examples:
SEX and SEX 2.0, stock iron: 30 henries, 8Kohms, nominal 0.9uF, range 0.45 to 1.8uF (ships with 1.0uF)
SEX 2.1, stock iron: 40 henries, 8Kohms, nominal 1.25uF, range 0.625 to 2.5uF (ships with 1.5uF)
SEX, SEX 2.0, and SEX 2.1, Magnequest upgrade iron: 50 henries, 8Kokms, nominal 1.6uF, range 0.8 to 3.2uF. Recommended 2.0uF.
