How to step up voltage on a stompbox or other European device
From the description in the question, the musical devices use individual external power adapters aka wall warts. The statement "probably are from 220v AC -> 9v DC (or 12v)" is not definitive enough to go on.

The recommendation is to measure the voltages at the connectors going into each musical device, determining both voltage, and whether it is DC or AC. Assuming they all use DC is not valid, many devices are powered by low voltage AC from a small wall transformer. Also, the power adapters supplying the musical devices probably have markings indicating their input (110, 220 or universal AC) and output (5, 6, 9, 12 etc Volts, and DC or AC). 

Then, simply replace each of the power converters for the DC devices, with "universal" wall warts of the correct DC output voltages. Universal = They work across the entire range of mains voltages, 100-250 VAC being common. 

For the devices that receive an AC input, say 9 or 12 VAC, replace their existing transformers with transformers with the same rated output voltage and current in each case, but designed for 110 VAC mains.