(setting the stage first) In a series feed SET amp, the inductance of the output transformer causes the impedance presented to the tube to drop below some low frequency, usually in the 20-30Hz range. This shifts the load line so that the tube becomes current-limited, in creasing distortion as well as limiting the available output power. The bass falls off at 6dB/octave below that corner frequency.
(now the parafeed difference) In a parafeed amp, the plate choke has the same effect, but the parafeed capacitance raises the cap/output transformer/speaker impedance at the same time, and if the capacitance is chosen well this will maintain the impedance seen by the tube down to a somewhat lower frequency - maintaining both power and low distortion. It's not a lot, at best something like a half octave, but it's still a good thing. Below that corner, the bass falls off faster, 12dB/octave. No free lunch, you know.
By increasing the capacitance, you move the situation closer to that of the series feed. Fortunately, the parafeed plate choke often has more inductance than the average series feed output transformer, so when the capacitance is very large it just acts like a high-inductance series feed design. FWIW, this seems to have been the approach used by Japanese audiophiles when they started reviving the parafeed approach, 20 or 30 years ago.
All of the above analysis is based on a pure resistance load, i.e. nothing like a real-world loudspeaker. That's why I so often recommend experimenting.
