How much is the voltage dropping when the Air Conditioner comes on? Does it give you a read out?

The Monster Cable Power Pro 900 is a surge protector only, it doesn't prevent voltage sags from reaching your gear. It has a filter to reduce noise coming in thru the power lines, but is that enough for the transients coming from your AC unit? I don't know.

If the voltage sags so much that your MODX malfunctions, I'd have an electrician look at your wiring. Have you tried measuring the voltage at the outlet when the AC kicks on? Also, try measuring the voltage coming out of the MODX's power adapter to see if it drops as well. I would think the power adapter is regulated, but too much of a power drop could cause the output voltage to drop.

Also, have you confirmed it's your MODX that is distorting and not your mixer, amplifier, monitors, or something else in the signal chain? If you plug headphones directly into the MODX do you still hear the effect?

BTW, my MODX's power adapter says it accept 100-240V, so it's clearly a switching mode regulated power supply which shouldn't pass a voltage sag to the MODX unless your outlet's voltage drops below 100V.

Are you in the US where the normal wall outlet voltage is around 120V, or are you in a country with a different voltage (like Japan, 100V, or UK 240V)?

That's a really good point about the synth versus the amp. First thing I will check tomorrow.
BM, no readout.

Yes, I am in the US.

Thanks, guys!

It is important to figure out which components are dropping out. But, in general, if your power integrity (of anything) is impacted by the A/C cycling - you've got a problem. I'm surprised the A/C is on low-line vs. high-line. Wall units use the same low-line for the "smaller" units. But central A/C would typically use a different circuit entirely than your keyboard and associated gear. If you're starving your main feed that's a "major" issue. There's certainly more diagnosis that can be done here - but it sounds like an electrician may need to be called in.

If you want to place a band-aid on this, you could use a UPS to deal with the brownouts. Sine wave may or may not be needed for the device(s) you're trying to protect.