[sdiy] Ladder Filter - dark side

[rob]

Hi
Having just built a Roland version of the Moog classic transistor ladder 
low pass filter....I have a problem or a unique feature..

The frequency cutoff works fine, turning it down cuts off the output signal.
But turning it even lower goes past complete cut off and then starts to 
increase the output but to a much lower signal level.
The beyond cutoff signal is 10x times lower in amplitude, same as the  
input frequency, but a sharper wavform.

Any ideas on what causes this behaviour?
Is it the CV control signal being too strong?
I have to admit the transistors (beyond the base pair) in the ladder are 
not matched.

Regards
Rob