I then tried my default, which did not work.
You did not say this in your earlier post, and it was the single most important detail of the whole thing. In fact, the other stuff is all noise. I don't care about what modifications you made to the DSP, right now. The initial point is to figure out if there is a difference in versions of MC, or if something else happened (upon which we can investigate the other tack). So, the rest hurts, doesn't help, because it introduces variables.
Not trying to be a pain, that's just the way it is if you want to solve problems.
So...
Then, to be clear...
1. When you made the initial post, you were running 19.0.128. (I verify this because it is possible that the updater could have downloaded an older build, and then installed that, even though 128 was released. This happens.)
2. Prior to the update, you had settings in the DSP and they worked. Post-update, they didn't work.
3. Normal stuff like rebooting and whatnot didn't solve it.
4. You rolled back to 125, and that didn't fix the issue.
Are all of those things true?
Then, either:
1. The timing of the upgrade and the failure to play back were either coincidental, or it somehow triggered a condition external to MC.
2. The change in behavior of MC is older than build 125 (which is a fairly recent version, and you might not have had it yet).
3. You aren't accurately replicating your settings, or some setting changed and you didn't realize it is the cause.
#3 is easy to address. MC makes automatic Library Backups. Restoring one of these backups, including settings, will put the DSP back exactly as it was. So, make a new Library backup (to save where you are now). Then, look in your Library Backup directory (which is in My Documents\J River by default) and find a backup from when you know it worked, and restore the Settings (you don't have to restore the Library which isn't related here).
#2 might entail going back further.
#1 let's not worry about until we eliminate #2-3 as potential causes.