I think in a sense both. The two "gain knobs" I would consider volume control knobs, one for each channel. The "output knobs" will allow you to adjust the output voltage from 0 to 30 volts for each channel, so that will regulate how many volts are controlled by the "gain knobs."

Playing with the correlation between the "output knobs" and the "gain knobs" should give you an interesting range of subtle sonic changes.

Here is Steve's initial setup guide for the output and gain knobs in the manual for the original CSP3:

Besides its incredible sound, what makes this preamp unique is its adjustability.  You can adjust the output level for each channel to anywhere between 0 and 30 Volts RMS which is enough to drive any type of amplifier with amazing weight and control.  It's a simple process.  You just set your volume control to 12 o'clock and then raise the output levels slowly until you reach what you would consider your normal listening volume.  Your preamp is now perfectly set and matched to the amplifier.

Glad I could help.