check the connections. check the cables. check the jacks. you say the guitar works fine by itself so its not your pots/jacks/wiring on the guitar, so it must be something like that with your pedal set up.
I assume this is a no shit statement but you need to run each pedal by itself, individually to check which causes the feedback. I would suspect its the wah, but you mentioned some trouble with the boss dh1..
if its the wah perhaps it is a wiring/jack issue with the pedal itself? you need to check out the hardware on the wah and yr other pedals to see if you can find the mechanical source of your error. This is the very reason why I like multi-effects in the first place, but then again, it is very ver difficult to find a multi-effects that sounds as good or gives you the versatility and freedom of expression as having a pedal board..
if its not an individual pedal causing the problem by itself, perhaps it is the set up. Maybe a knob/setting was accidentally messed up adding to much gain or treble etc, and this is adding feedback to the chain. So if you do not find the culprit by checking each pedal individually (NOT IN CHAIN, BUT BY ITSELF guitar->pedal->amp) then begin to add the chain together one by one, adjusting knobs and settings to see if it reduces to feedback. Uncontrolled feedback generally results from one of three things:
a) a wiring/jack/pot issue with the guitar or the pedal (perhaps the gain/vol pot on one of yr pedals is burning out, thats what happened to me with a similar situation)
b) a setting issue with a gain or volume knob set to high (or too low) in the chain
c) a set-up issue, the pedal is just not in a nice order for yr desired sound. shuffle the order of your chain around and see if that solves the problem as well..
I say its probably a, I would suspect if it remains mysterious that its a pot issue.
you might have to take apart yr pedals and examine the wiring/pots inside for problems..
best of luck.