Op-amp question/problem: squeals when I turn down guitar volume

[earthtonesaudio]

I've got a fairly simple circuit on the breadboard, and it is fine except it howls when I turn the volume down at the guitar.  It's a twin-T wah circuit, using a dual op-amp.  The first op-amp is just a non-inverting buffer, 1M input Z.  Second opamp is used like this:
http://www.geocities.com/tpe123/folkurban/fuzz/simplevcf.gif

The only thing that seems to remove the howling is cutting the output volume, but that brings it below unity compared to bypassed.  So what should I do?  I want to stick to the 2 op-amp design.  Makeup gain after the filter stage?  (I put the buffer there initially because I thought the filter was being loaded down at the input...)

Thanks in advance!

-Alex

EDIT: opamp is a TL072