Confirm the type of channel switching the amp uses, either from the amp's manual or manufacturer to make sure you won't damage either unit.
From the manual, section 4 Relays:
"IMPORTANT! The relays of the FX8 are designed for use ONLY with amplifiers that use “short-to-sleeve” type switching. Do NOT connect the FX8 relays to the switch jacks of an amp that uses voltage differential switching or any other type of switching aside from short-to-sleeve, or serious damage can occur to both units. If you are not 100% sure, contact your amp manufacturer to determine whether your amp is compatible with short-to-sleeve switching. The FX8 relay jacks are compatible with TRS cables, TS cables, or TRS-to-dual-TS split cables. The relays are also fully isolated from the electrical ground of the FX8.
This warning is stern, because the last thing we want is for anyone to damage their amp or FX8. In fact, short-to-sleeve relay switched amps are quite common, and your amp may be perfectly compatible. We need to require you however, to understand how your amp works and make the right choices about connecting it to the FX8 relay jacks.
Your amp manufacturer should be able to help.
The FX8 features two TRS (Tip-Ring-Sleeve) relays that can be used to switch the channel or other functions of a connected amplifier or device. (Wondering what TRS is? See the FAQ on page 17.)"