I don't have a D yet, but your "D" file sounds good; the sort of high freq loss I would expect from a lot of compression with just about any comp. Maybe someone who's build a D or owns a real on can comment on that.
I did notice something unique about the "A" file... a distinct asymmetrical distortion on the top half of the waveform. Zoom in on it in any wave editor (I used WaveLab) to see what I mean. I'm gonna say this is either happening in the signal amp or the line amp, either Q3, Q5, or Q6. Also make sure CR1 is the right way around, with the line (negative "cathode" side) towards the outside edge of the board. Something is causing the positive swing to be clipped or something. Could be a bad transistor, or maybe (someone tell me if this is wrong) a decoupling cap nearby not providing enough current and dragging the +30 rail down on loud notes. And I know it sounds redundant, but double check all of your voltages per the volts schematic. Check every gate, source, drain, base, collector, and emitter to be sure, because that would have been my first guess. Like an improperly biased transistor. Sort of like the way a fuzz pedal distorts half the waveform.
If all the voltages check out, fire up the scope and feed the 1176 a strong input. 1kHz sine is best, 1v or there abouts, but anything will do really. My old wavetek puts ou 1v and I know this can sometimes be too much, and I have to pad it down. Square wave can be useful in situations like this too, but here you should start with a sine wave since we're looking for asymmetrical distortion and it will show up best as an asymmetrical sine wave.
Inject the signal with the 1176 on and in/out at 12 o'clock or less (less on the input? maybe 9 o'clock, you'll adjust as you go, you'll know when it's too hot or too quiet, the scope will show you), or wherever typical operating ranges are, ground the scope probe's ground clip to PCB ground (any ground, just make sure it's ground!) and probe the input XLR pin 2 to see signal coming in. It should be clean and sinusoidal. Use this opportunity to set up and tweak your timebase and volts settings on your scope, focus, intensity, position, volts/div, timebase, trigger, ac couple, etc, bringing the picture into view until it's stable and clear.
Now follow the signal step by step, next moving to where the signal exits the input attenuator, then moving to the input transformer's output, and using the schematic to probe off componant legs (or underside of board, which ever is convenient) step by step tracing the path though the signal amp, output pot, line amp, and finally through the output transformer and output XLR.
At some point the waveform should become distorted, one half of it looking weaker or filtered looking. This should give you a better idea of which section is in question.
All comments and corrections welcome (i'm still pretty noobish here too
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