First of all welcome to the forum! The Maz 38 1x12 was my 1st Z some 9 years ago - killer amp, but I'm glad you got the newer light weight one. They are/were heavy.
Re the trouble shooting I would 1st try to narrow down where the issue could be.
Try playing the amp without the pedal board and see if the drop off still occurs.
Check your cables as well - maybe a cold solder joint in a patch cable or instrument cable.
Try different guitars as well just in case.
Check the speaker connection to verify you have a secure contact.
If you have an effects loop on your amp spray contact cleaner (DeOxit is great) on a 1/4" plug and insert and remove few times. Your amp is newer but oxidation in the loop has been known to cause strange sonic occurrences.
Also check your tubes. One at a time remove a tube then reinsert it - all the way down the line. It doesn't sound like a micro phonic tube issue but again you want to rule out everything. With an older amp you can spray the pins on the tubes before reinserting to remove dirt or oxidation. This is also a great way to keep your cables or any input jack clean.
If you still don't resolve you problem you can email Z support and they are pretty quick in getting back to you.
Good luck, and keep us posted on your results!