OK, so the Stomp Box MkI didn't quite work out as planned. I didn't expect it to be perfect first time. One-off projects like this evolve over time.
The first fault was with the LED power indicator. Personal memo to self: never connect a self-blinking LED across the power input of an audio device, unless you actually want that Floyd-style heartbeat in the background all the time.
Second fault... I had such faith in that piezo transducer! I was so certain that, being a contact device, it would be relatively immune to external noise and would pick up only the stomp from the box. I was right, but in such a wrong way. No feedback problems (hurrah) but every last detail of sticky shoe sole issue was faithfully fed to the amp, making it sound like an octopus making love to a biscuit tin. Boom-schluppp Boom-schluppp, etc. In desperation, I added a layer of rubberised carpet to the top of the Stomp Box. This made it sound like an octopus trying desperately not to be discovered making love to a biscuit tin.
The blinking LED had to go. I replaced it with one of those low power plain jobbies with a built-in resistor, designed for direct connection across rails. So far, so good.
The piezo element had to go. I replaced it with a dynamic element from a crap old cassette-recorder microphone, embedded in foam packaging.
As far as I was concerned, I now had a Stomp Box! I shoved it through my 100W Laney Linebacker, took the top-end off and was pleased to hear a satisfying Whump Whump Whump coming through.
However, everyone's a critic, so it seems. On its first night out, the Stomp Box was still deemed to be too "toppy". Some said that thicker carpeting or even neoprene rubber casing might help. Others argued that an active low-pass filter might be the solution (sledgehammers and nuts spring to mind). Personally, I suspect the ability of the performers. Just because it's called a Stomp Box doesn't mean you can just plug it in, stamp on it and expect to sound like Catfish Keith on day one. These things need learning...