If you connect a mono stomp box such as a Boss GE-7 using a single 'SEND' & 'RETURN' input, you only get the stomp effect through one channel (ie one speaker) with the other channel (& the other speaker) delivering the 'ordinary' tone.
The only way to keep the amp in full stereo mode is to either: -
1. Use 2 stomps - one through the LEFT SEND/RETURN, & the other through the RIGHT SEND/RETURN. This isn't very practical!
2. Alternatively, you could use a stereo EQ unit (Behringer sell an inexpensive one). Again, not ideal if you already have EQ stomps that you like (eg I have a Boss GE-7 & a Behringer EQ700).
PART SOLUTION
Connect a regular guitar lead as normal from your (mono) external EQ 'input' to the LEFT SEND in the stereo FX loop. Use a MONO (not stereo) Y cable to connect your EQ's 'output' to the AD120's two RETURN jacks in the FX loop. This will give you EQ through BOTH channels (ie both speakers).
The difference in tonal quality will be HUGE as compared to using the EQ through a single channel! This is now the set-up that I use. I can't believe the difference or understand why I didn't suss it was only coming through one channel before. With my AD212 connected, I now sound like I'm playing through a Marshall stack!!
The only minor downside is that the stereo effects on your amp (Rotary & Chorus) will be MONO. Bearing in mind that with the combos (such as my AD120VTX) the speakers are too close together anyway for any real stereo spread, the differential is minimal & virtually indiscernable - unless you listen really hard with your ear to each speaker, you won't notice any difference at all!
If however you have the AD120VTH and are using separate cabinets for each channel, the difference will possibly be a little more noticeable & if you are really into your stereo effects, you might prefer to use a stereo EQ.
Now, you might be asking 'what if I use a Y cable at both ends?' of my mono EQ. My best understanding from our forum discussions is that this will not make any difference to the loss of stereo effect. I am not 100% sure, but there's a valid reason that I don't want to try this. Nik (who is an electrical engineer & has studied the specs) has suggested that this MAY overload the FX Loop specifications & could give rise to 'difficulties' - & I have no wish to put my amp under any undue 'strain' and possibly risk something blowing!
If anyone has any further authoritative information on utilising 2 Y cables through the FX Loop, please do contact me.
But for now, I'm happy with the single Y cable solution. The sound difference is enormous and way, way compensates for losing any stereo effect.
One thing that we (and various in-store techies) agree on, is that the stereo FX Loop design is just plain daft!
As a comparison, I had a Line 6 Flextone II Plus & 1x12" extn cab. This utilised a single LEFT & RIGHT socket in the FX loop, but it is a stereo loop. When connecting a mono stomp/efects unit with a mono lead, the external effect is mono, but comes out through both speakers. When connecting a stereo-effect with a stereo (TSR) lead (Tip, sleeve, ring) then the effect is true stereo. Why the hell Vox didn't adopt this approach, goodness knows! 
Hope this update is helpful!
Rich
