By popular request, here are my You-tube video links demonstrating the tonal difference that an EQ in the FX loop gives you.
Also, read the 'Understanding EQ' FAQ section too - AD120 owners should read the 'update' on how to connect the EQ with the stereo 120's ('Y' cable needed).
All my You-tube vids are here (or just search for 'voxman5' on You-tube): -
Voxman's You-tube video clips (all)
Within the above, there are two clips demonstrating 'with and without' an EQ in the FX loop: -
EQ in Valvetronix FX loop 1
(There's also a 'video-response' from Bodine1231 in the US - regardless of whether you like his high-end 'Metallica' EQ settings with scooped mids or not, you can certainly hear how the EQ can totally reshape tone!)
EQ in Valvetronix FX loop 2
All the other non-gig clips are with the EQ in the FX loop. On the gig clips, the EQ was only going through one channel, as I hadn't then yet realised that you need a 'Y' cable to go through both channels.