Hi johnw,
You are right that it can be confusing. In this case, since the preamp is applying a fixed amount of gain to the signal, it is ok to add the amount of gain it is adding in dB to the signal size in dBu to get the final signal size delivered to the recorder's 1/8" input in dBu.
The confusing part is that gain is just multiplying the signal size (Volts) by a number like 10. But when you express this in dB you ADD it (20 dB) to the signal size (also expressed in a dB scale: dBu). You are right that you cannot mix dB scales that have different references like dBu and dBV. In this case we are staying in the dBu scale from the input to the output.
The scenario you are describing is exactly the issue. If you add (in dB) too much gain to the mic's signal, you will make the mic so sensitive, that even moderately loud sounds will overdrive the MixPre 1/8" input.
Willndmb,
AKG at max SPL is too hot even if the preamp has gain=1 (0dB) but that doesn't mean you can't get a great recording since most shows are not going to be that loud at the mic location.
Here's the dB-SPL that it takes to hit the +10dBu level for the AKG mics (or the DPA 4060, or any mic with sensitivity = 20 mV/Pa) and NO additional preamp gain.
Gain = 0 dB (Pre set to unity gain)
AKG SPL @ 10 dBu Output = 136 dB SPL
If you are adding gain (in dB) in the preamp, you will need to subtract the gain you are adding from the max SPL you can expose the mic to. Adding gain makes the mics more sensitive so you can't expose them to as loud an environment.
Gain = +4dB (tb)
AKG SPL @ 10dBu Output = 136 dB SPL - 4 dB = 132 dB SPL.
Gain = +20dB (Nbox)
AKG SPL @ 10dBu Output = 136 dB SPL - 20 dB = 116 dB SPL
The MixPre 1/8" input won't work in loud environments with many mics if you add too much gain between the mics and the 1/8" input. But if you can keep the gain you are adding between the mics and the 1/8" input around unity, it should be ok for most mics. I guess as more people try different combinations of mics, pres, and dB SPL into this input, we will see...
MIQ