hopefully the staff can help me on this, i’m really frustrated.

i just got my nano muff back today, after sending it in to EH to be repaired. my problem had been that at full guitar volume, the pedal sounded fine, but as soon as i backed off the guitars volume, the signal plummeted and got all fizzy. i got the pedal back from repair, took it to my amp, plugged it in from my guitar to the pedal, then to the amp. cranked it up and everything worked wonderfully. backed off the guitars volume and it controlled the gain perfectly. all happy, i replaced it into my effects line: a boss tu-3 tuner, a “build your own clone” overdrive II that I built, and an electro harmonix holy grail nano. the holy grail and the tu-2 have adapter power, the ODII and the nano muff were running off batteries. the second i engaged the muff, it started to squeel, so I unplugged it and went back to just the nano in the signal line. blown again. same exact problem. i am so frustrated at this point. what could possibly be ruining my nano muff, and will it ruin other pedals i put in my signal chain?? gah!!

thanks for any help