To make sure which side is really the few DB lower, try swapping both inputs and outputs from left to right, that is, left out to right next-device and visa versa. Do inputs and outputs separately and check between each step.
The problem should follow the swapped direction and now manifest itself on that side.
When you've made sure which channel is the culprit, as Nolan says, try cleaning the sockets.
If this doesn't help, open it up, inspect, and if nothing seems obvious, try locating which opamps sits in the signal chain.
If those are in sockets try pulling and reinsert them a couple times; chances are there could be corroded contacts.
If this doesn't help, try re-soldering first input and jacks, and if still no luck, then re-solder the whole circuit board.
Do you know if there are programmable analog potmeter IC's in the signal chain?
If so, those too could be either partially defunct or have a socket/solder problem.
Disclaimer: I have no experience with the SX700, and never seen the schematics.