Not sure most Piezo pickups have a preamp.
Try it with any guitar pedal and if it works you will be fine.

I see that you’re happy with the sound. Testing it yourself is always best, but for future information, you definitely want a high input impedance with piezo pickups. The higher the better, especially if you’re not using a preamp. Without a preamp, you want to make sure that the pedal has a high gain control. With a preamp, it’s not so much of a problem because the preamp is providing gain, but you still want your piezo to “see” a high impedence so that it won’t lose a lot of low end. The Pocket Metal Muff has a 1MΩ input. 1MΩ [strong]is[/strong] a high impedance, so the short answer is, “Yes. It will work well with your piezo pickups.”

Also, the Pocket Metal Muff is true bypass. That means that when you switch it off the input jack is directly connected to the output jack. Most people are looking for true bypass, but this is NOT ALWAYS best. There are many situations where a buffer helps a lot. The Micro Metal Muff has buffered bypass. That means that it is always affecting the signal, even when the effect is off. The Soul Food has internally switchable bypass so that you can pick whichever sounds better – buffered bypass or true bypass – in your signal chain.

You were right to ask about the Pocket Metal Muff because not all overdrives have a high impedance input. Most have a true bypass which means they will not affect your signal when they are switched off.

Big Muff Pi Tone Wicker 130kΩ
Crayon 315kΩ
East River Drive 375kΩ
EHX Tortion 1MΩ
Russian Big Muff Pi 130kΩ
Micro Metal Muff 1MΩ, Buffered Bypass
Octavix 1MΩ
OD Glove 300kΩ
Soul Food 1MΩ, Internally Switchable True/Buffered Bypass
Tone Tattoo 1MΩ, Buffered Bypass

The EHX Tortion might be an interesting pedal for you because it has a high input, lots of gain, lots of tone control, and it also has a built-in DI. It would allow you to plug straight in from your piezo and wouldn’t require you to use a separate DI box. You could use it with or without the Pocket Metal Muff.

The Octavix would probably sound amazing with your violin. It also has a 1MΩ input, so it could go at the beginning of your signal chain like the Pocket Metal Muff.

The Tone Tattoo has a Metal Muff with a noise gate, a Neo Clone chorus, and a Memory Toy echo all in one box. Having a gate on the Metal Muff is very useful because any high gain effect is going to be noisy, but you can also use the gate to make the echos more distinguishable when you’re using the MM with the Memory Toy. The Tone Tattoo has a buffered bypass, but with a piezo pickup I think a very high Z (impedance) input with buffered output is what you want, and I think these three ALL ANALOGUE effects would sound fantastic with a violin.