You should troubleshoot both channels! In a phono preamp, the EQ pulls the signal down significantly everywhere except the low bass, so while my opinion is that you have a flaky solder joint on a ground wire in the offending channel, you could also have a bad EQ connection in the channel that seems to be good.
To measure the signal voltage, you will need a meter set to AC volts. Your meter will need an AC volt scale that's pretty low, most of them will have a 2V setting or a setting that looks like mV~. If you meter only has something like a 200V and 500V (or 200V and 700V) scale, then it won't quite get the job done. The $25-ish meter at Harbor Freight is perfectly adequate for this job.
To get your bearings, you can hook your phone up playing a 60Hz tone to the Reduction with the Reduction powered off, then crank the volume up all the way on the phone and measure AC voltage between the center pin and ground on each input jack. For most phones, this will be about 0.3V (300mV). For the remainder of the tests, you'll want to turn your phone down so it's at 10mV or less.
