Dual rectifier FX loop volume drop issue

Hi guys,

I have 1990's DR with a TC Electronics G Major 2 in the FX loop. I'm using CH3 with FX loop off for my rhythm and CH2 with FX loop on for my leads.
If I switch the CH3 FX loop on to use some delay/flanger FX etc. the volume is reduced. I assume that's I just because I need to set the loop levels to match the channel's volume. But if I go back to my rhythm patch and switch the FX loop off, the volume remains at a reduced level. If I then switch to CH2 (my boost/lead channel which has the FX loop on) then back to CH3, the volume is back where it should be. Has anyone seen this before or knows why it happens?
I've had the amp for about 8 years but this hasn't happened before. Someone suggested that it could be bad patch cable in the loop?

Cheers

My setup rig is as follows:
MESA DR + 4x12 oversized recto cab
Furman power conditioner
Korg Rack Tuner
TC Electronics G Major II (in FX loop)
G Lab MIDI 2x Looper->Ibanez TS909->Boss Acoustic Simulator (In front of amp)
Voodoo Labs Pedal Power

ESP EC-1000 Deluxe
ESP EC-1000FR Deluxe