Much do what I have seen you write is very sensible.
So I am thinking I may have missed some nuance here.
The specifications for "maximum output" of a few vintage active preamps of my acquaintance:
B&K MC-101 Sonata: 14 VRMS (as specified)
Apt Holman: 9.1 VRMS (as measured by Ken Rockwell at 0.1% THD; Amir didn't measure it)
Adcom GFP-565: 10 VRMS (as reported by S'phile), ("rated" output: 2 VRMS)
SAE P102: 7.2 VRMS (specified also at 0.1% THD), ("rated" output: 1 VRMS)
So, when you crank them up, they add quite a bit of gain above unity. These are peak values, meaning the highest RMS average waveform voltage without clipping. Distortion is higher, of course, but still reasonable (often under 0.1%). Any of these (except--barely--the SAE) can run that Benchmark amp in low gain to full power. Whether one would want to do that rather than use the extra input amps in the Benchmark is a whole other thing--the preamps in the Benchmark are likely much better than in any vintage external preamp.
When I press the switch on my B&K to defeat the line amp, I the voltage going out becomes identical to the voltage coming in from the source. And the volume drops
substantially. That is essentially the output level provided by the miniDSP, because it apparently provides no analog amplification. But, as I understand it, it also can't take more than 2 VRMS (unbalanced) coming into the analog inputs without clipping (4 volts for the balanced model served by balanced sources), because that produces 0 dBFS in the digital realm and digitally clips above that. There's nothing wrong with that for amps that have enough gain.
I've ordered a couple of those Sparkfun Outsmarts balanced-out differential amplifiers that use the THAT 1647 custom differential op-amp. It's noise and distortion is well below my preamp, and it provides 6 dB of gain (because it is doubling the single-ended voltage to a true differential balanced output). That means the 2-volt sources will produce 4-volt balanced outs, which will drive both the miniDSP and my amp comfortably. My preamp seems to have the necessary +/- 15VDC needed to accept and deliver enough voltage to sit in the output of the preamp even when the line amp is active. That sounds like a fun project to me whether or not I buy the miniDSP when the balanced version is again available.
Rick "there's line level and then there's line level" Denney