A couple of things may be going on here...

Many Y adapters that the stereo single and mix it down to mono for each side, meaning each mono jack gets both tip and ring signal.   You have to make sure, on your particular Y adapter that the stereo tip goes to one jack and the stereo ring goes to the other. 

The FX loop on the DC30 is a simple interrupt loop.  It's pretty much a hack.  All it does is split the circuit right before the phase inverter and allow it to be sent out the FX loop and then back again before continuing on into the power amp.  It's signal may be too hot for pedals, which are expecting to see guitar-level signals, and too unstable even for some line level racks systems.  Interrupt loops are a simplest way to add FX loops to circuits because they don't add additional components to the signal chain, which is why you usually see them hack onto amps that were designed with no intention of an FX loop.  Since the DC30 is just a trem-channel-less AC30 with more filtering, it's fairly bare bones, the FX loop was added practically as an after thought.  And the reason long cable runs don't work well is because you are sending all the signal from the amp out through the FX loop and back again, in short you are adding an addition 20' or 30' of wiring in the signal chain of the amp. 

Now, with all this said, modern amps, designed to have FX loops, almost always use buffered FX loops...buffered either with extra tubes or through op-amp chips.  This allows both guitar and line-level FX to be use in the loop.  Dumble, who commonly used the interrupt loop used to build a tube-based outboard buffer called the Dumblator (sp??).  Ceriatone offers a SS version called the Klein-ulator. 

Sometimes, in the past, I've been able to get by with just having a buffered effect (Boos, Ibanez, DOD, etc) as the first and last thing in an interrupt loop.  For a while, I would use an old DOD Gate/Loop pedal ($20 or less used) and put everything for the FX loop in the DOD's loop and let that one pedal provide the buffers at both the beginning and end...in short I used it the same way one would use a Klein-ulator.

But whatever your solution is, you'll have to do some testing to figure it out. 

Thank u very much for that thorough explanation! 

I had been reading in the interim, and I had found out about the Dumbulator; but I just could not fathom why the Dumble amp would need such a thing.  Seeing this effects loop as a "hack" as you put it, it becomes clear why just running it out to my pedal board will not work, probably ever, unless I had some kind of buffer.  Now then...

The idea of a buffered pedal sounds, um, antagonistic to the concept of preserving tone, am I wrong about that?  I've spent all this time getting custom or modded "true-bypass" pedals, and now, ironically, I need something that sounds, well, opposite to that approach... is there a pedal that just acts as a buffer and nothing else?

Last subjective question, I hesitate to open this can or worms:  what would be the impact on tone to add either buffered pedals, or to add a buffered FX loop to the circuit?  I've read some people say FX loops ruin everything, but I've owned some great amps over the years which had them.  What would this addition do, in your opinion, to THIS amp?  any / all comments welcome...

thanks again!

Oh sorry... I mean a TimeFactor delay pedal. This can act as a buffer as it has input/output level switches (line/amp/guitar) that vary the level incoming and coming out from the pedal. There is a always an slight high freqs cut if you a/b the straight dry signal to the amp and the dry signal having the loop connected (pedals off). But I compensate this by opening the cut control. I use the loop basically for the delay pedal, and an volume boost for solos via en ge7 moded eq.
The signal comes and goes by one stereo cable, so I am saving one cable on the setup!

Hope this helps!