Most MFX already have internal voltage regulators, so most unregulated DC adapters will do. BUT some DC adapters do not have enough DC filter capacitor (low value capacitors ~1,000uf) to reduce hum. The solution is to add or replace the existing capacitor with a higher value capacitor (2,200uf or higher). If your MFX is big and uses a lot of current, use a 4,700uf capacitor. After doing that, try operating the MFX at 6 volts. If it works at 6 volts, use that setting. If it doesn't work, move up to 7.5 volts but never go beyond 9 volts. If it doesn't work at 9 volts, use a higher rated adapter (1000mA or higher). If you use 9 volt setting when your MFX works at 6 volts setting, the extra voltage-current delivered to your MFX will only translate to higher heat dissipation form its internal voltage regulator which can damage other components near it and/or melt plastic casing.
My Zoom707II works at 6 volts setting with 1000mA rated adapter and 9 volts using a 500mA rated adapter.
On old (read: classic or vintage) stomp boxes, it's better to use a well filtered and regulated power supply because many of these only use simple zener-transistor voltage regulators.
