Fuzz and clean DI problem

This is a strange one...

I have a problem getting a clean DI signal when using a fuzz pedal.

I want record my fuzz driven preamp and a DI signal at the same time to my audio interface.
But, when fuzz is active, my clean signal drops in volume ( about 15-20 dB ) and distorts. Not usable at all. Driven signal is unaffected.
When fuzz is not active, everything works as supposed to.


Signal chain:
guitar ( ESP baritone with passive humbuckers )
Lehle p-split passive high impedance splitter ( two high impadance outs; dir. and transformer isolated )

signal 1:
from p-split dir. out to Zvex fuzz factory pedal ( battery powered )
Peavey Rockmaster tube preamp guitar input ( powered by 110 to 230 Volt transformer isolated converter )
Peavey XLR-balanced out to Digidesign Mbox 2Pro line input ( pin2 is +/hot )

signal2:
from p-split isolated out to Mbox 2Pro Hi-z input


I´ve tried:
-changing cables
-another fuzz
-uncabling the xlr-signal going from Peavey to mbox
-powering fuzz with a transformer ( mains power )
-inserting a passive DI box after p-split ( p-split iso out - DI xlr out - mbox 2pro mic in )
-to replace the p-split with a passive DI box ( DI xlr out to Mbox mic in, DI through to fuzz )
-to replace the p-split with a Lehle 1at3 splitter ( out A to Peavey, out B to Mbox )
-inserting the p-split after Lehle 1at3 ( out A to fuzz, out B to p-split, iso out to Mbox )
-inserting Lehle 1at3 after P-split ( P-split dir. out to fuzz, iso out to Lehle 1at3, out A to Mbox )
-another hi-z input on Mbox
-another line input on Mbox
-flipping ground switches wherever there`s one
-flipping phase switches

...and nothing of the above helps.
Any ideas?
It must be a somekind of an impedance thing, but how to solve this?
Another audio interface?

Thanks.