It appears to me that the input buffer is at unity gain, while the tube amp provides 12 dB of gain. But it also appears that you can run it 1.) passive (neither input buffer nor tube amp), 2.) with just the input buffer, and 3.) with the tube amp. Based on that understanding, I don't think you can be sure of "unity gain" unless you select one of the first two of those options, for both of which the volume control is at zero attenuation when turned up all the way.

So, a 4-volt source in the first two options would produce 4 volts at the outputs.

The 1ET400A has 12.8 dB of gain, and doesn't reach 1% THD until the input voltage reaches 9.6 volts RMS.

4 volts plus 12.8 dB of gain equals 17.6 volts. (Voltage doubles every 6 dB.)

The F208 nominal impedance is 8 ohms, but much of it is less than that. We'll go with that. 17.6 volts into 8 ohms is 39 watts. The F208's are 89-dB/W/m efficient at one meter, so a signal at that voltage level will produce about 105 dB SPL at one meter. (Power doubles every 3 dB.) Two speakers will add 3 dB. That will make my ears ring if the average level of a song is close to that peak. Even if the song's peaks are 15 dB above the average, though, it will still be quite loud.

The Purifi module is good for 180 watts at less than 1% distortion into 8 ohms, though that is at the brink of clipping protection. Running it at full power would increase loudness by 7 or 8 dB--bring your earplugs if what you were listening to was loud. That would take 38 volts at the speakers, or just about 8.7 volts at the amp's inputs. Again, 7 or 8 dB of gain needed by the preamp to achieve full power.

Rick "calculating in haste" Denney