If the spec is accurate, the level difference between dBFSD in REAPER and analogue dBu on the console should be about 21 dB -ie if you send a steady signal (eg sine wave) that peaks at -10dB in the REAPER channel, it should measure +11dBu unprocessed at the console channel.
If you like you can think of this as the DA Converter sensitivity, 0dB digital in is +21dBu analogue out, -21dB digital in, 0dBu analogue out (I haven't been able to find any figures to the contrary in the manual, though I suppose it could be 20dB for round numbers). Remember the numbers are measuring different things (digital/analogue).
Any valve drive is likely to affect the channel gain (metering is AFL, not PFL), but it should be quite close. If the drive knob doesn't seem to act like a gain knob then I'm presuming it is gain compensated.
In this case, if there us a 20, 21dB difference in the numbers, you have unity gain (gain here is the number of dB added or substracted after adding 21 for DA conversion). The level knob should be reasonably accurate, providing the drive circuit isn't boosting things. You haven't got that "boost" button pressed have you?
beingmf - are you talking about compensation for RMS or are you anticipation pan law here. I was keeping it peak metering all the way as REAPER channel meters are peak and the GSR channel meters are peak (except for the LR VU panel meters).
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