FWIW I i now tend to use my trance inducer contact mic with the BB preamp
its what works for me - I definitely like the BB preamp having XLR balanced output
but I am less impressed with the BB contact mic element
I am sure it is fine if all you want to do is stick it to a piano
(& a zither isnt too much different to a piano)
but I want to stick it to all sorts of random stuff
some clean & easy... some messy & difficult.
& the trance inducer mic seems to translate cleaner bottom end,
so I use it (with a Neumann KMR81 short shotgun on the other channel)
I havent tried a C-Ducer but bear in mind it isnt a spot contact mic
its more like a 10cm strip of plastic, so only applicable in certain circumstances....
experimentation is the key.
contact mics need good impedance matching
& lots of clean gain from the the recorder AND the DI/preamp.
its quite easy to get some kind of signal through,
but to get a clean, dynamic, high resolution signal from an object
is purely on a case by case basis....
so best to keep an open mind & be surprised by what you find!
eg my old studio had a big metal gate (3m x 5m) in the yard
that creaked like crazy when it was very dry & swung slowly...
so i tried sticking my contact mic near the hinges, which is where
my ears told me most of the sound was coming from...
but the contact mic doesnt hear what your ears hear & putting it by
the hinges sounded weak.... it was only when i attached it at the
place with the most vibration (a diagonal strut) when i got really
strong resonant sounds....
Its almost better to listen with your fingers before you attach a contact mic...
and yes, making a contact mic stick to a rusty old gate is a PITA
but perseverance provides its own rewards
FWIW there is a US$3000 contact mic made by Sanken
that can 'hear' ants feet - people use it to detect termites
but similarly, its application is on a case by case basis:
put it on a comparatively LOUD zither & it will probably
distort and/or break..... put it on a big metal gate
& you better be the worlds most subtle gate opener!