Hi hylwelg,

Imerkat is right.  A buffer or DSP bypass is the way to solve the problem.  While cable length can influence the problem, the real culprit is impedance mismatching.  Passive guitars have low output, but higher output impedance.  You need an amp or pedal input with even higher input impedance to make sure that the majority of the signal is "sunk" at the destination and not the source.  Active guitars solve this b/c most onboard pre-amps have very low output impedance, but I don't necessarily like them (other than basses).

The whole "true bypass" thing, in my opinion, is misleading.  It would seem to imply that the most "pure" version of your signal is making its way to the amp, but this is not the case for instruments with passive electronics.  In reality, running a small, high output impedance signal through a bunch of "hard connected" boxes just creates an ill-conditioned signal path for passive instruments.  In your case, without a buffer, the signal is being loaded down at the source (in a frequency dependent lowpass way), the guitar.  The buffered solution actually preserves the signal better as it "opens up" the path for the signal, so to speak. 

Russ