What causes this behavior? Bad output cap?
My Boss MT-2 Metalzone is behaving strangely on fast cutoffs and to me it looks like the effects of a large cap discharging or maybe a leaky cap. My 5150 exhibits this same characteristic when I take a recording from its Preamp Out, whereas it does not do this when recording from the FX Send instead, so if I figure out what it is I can fix both of them.
Here is a comparison of my Japan HM-2 at the same section using the same source (a DI reamped) To me, this looks to be behaving as expected.
Is this a problem that has just developed on the two units? Was there an audible problem that caused you to investigate the waveforms, or is it that you see the outputs from the two sources appear different from the third and you are looking for a reason? I agree that it does look as though there may be a vertical displacement in the waveform that is then high-pass filtered, but it could be due to a non-linear effect in the pedal such as partial rectification rather than a leaky coupling capacitor.
How have you captured those waveforms? They look a bit strange for a re-amped guitar, but maybe they are normal for waveforms in your genre.
It is not linked to a sudden audible change to my knowledge, it's just that I am looking at them. These waveforms may look off to the eye because these are not speaker/mic level audio, this is a straight reamp of the pedals only directly back into the board through my Countryman Type 85 DI. My speaker/mic recordings do not look like this, they look as expected. The MT-2 sounds fine to my ear, but maybe it has an issue and could sound better? I don't know.
I know that my 5150 displays the same characteristics when I take preamp recordings of it. If I take the preamp tap from the "Preamp Out" on the head, the waveform at cutoff looks like the top picture here, it displays some drift that looks wrong to the eye and isn't present in the input signal. When I take the preamp tap from the FX Send instead, it looks more like the bottom picture here, much cleaner without any unexpected waveform drift or artifacts.
Recording a distorted guitar direct through DI is so harsh and poor sounding, I'd never consider recording without a mic (or 2) in front of a cabinet. Unless you want to use an amp simulator, in that case, the hell with the metal zone, I'd choose an Ampsim with all the pedals I need from inside the software.
Now to the why ? I guess distorted circuits may give some DC peaks when you stop playing. They are kind a made to boost signals so no surprise there. If you put and electricity meter, you should see a difference in voltage. I wonder if it could even damage A/D converters or other units, if plug directly. So be carefull, not to be bored enough to try fullish things .. (joke) ! ;)