Time to simplify the patching and trouble shoot each bit. Maybe start with just guitar <> MP1, un-patch the loop (the MP1 loop is serial so that could stop the signal getting through properly if there are lead issues or MXR issues) try output B to power amp with good known patch lead. Check if the headphone out is working (plug in some headphones). It's probably time for some new tubes, they are easy to change but I'd try all the above first. Are you running in mono ? or stereo ? We need to work out if it's an issue in the MP1 but it may be as simple as a patch lead. If you have low signal with just guitar <> MP1 <> headphones the next thing I'd try is taking off the lid (easy, bunch of small philips head screws, careful don't loose them or over tighten when you put the lid back on
), then clean all the pots and jack sockets with some decent contact cleaner (I use CRC CO Contact Cleaner), you can clean all the jack sockets without taking off the lid, give them all a decent spray. This is all worth doing from time to time regardless. To clean the pots, use the little plastic tube that the contact cleaner has and spray into the pot where the connections go in (should be a small gap) while turning it full up down a few times (this will clean any residual carbon etc from the pots sweeping contacts). ADA recommended a tube change every 2 years. In practice the tubes tend to last much longer depending on usage and climate. If the tubes just have red 12AX7A on them and no brand they are probably the original Chinese tubes and are as old as the unit.
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I also noticed when I wiggle the chord going from output A to my power amp the wrong way I get signal loss) this shouldn't happen and is probably either the patch lead or the jack socket(s) needs a clean. Also, given how cold it can get in Chicago, you need to let the tubes cool down (~15 mins or more) before going outside or they will die faster (and could fail). Let me know how you get on.