Multi-FX processors can be connected in a variety of ways for a number of different applications. Lets look at a few standard setups:
Multi-FX units featuring amp simulation/distortion are often used as the main building block of a finished guitar sound. In this role, the unit is simply connected between the guitar and amp or PA system. When using a guitar amp, speaker simulation should be turned OFF. The amps controls should usually be set as flat and clean as possible to avoid introducing distortion after effects such as chorus, as this usually sounds pretty bad!
Front End (as preamp)
To avoid introducing unwanted colouration and distortion, the unit may be connected straight to the power amp input of a head or combo, bypassing the preamp. Some amps have a dedicated input for this purpose, although connecting to the return socket of the FX loop usually has the same effect. A dedicated guitar power amp may be used instead of a head or combo, but in either case, if a guitar speaker or cab is used (as opposed to a full range speaker or PA system), speaker simulation should be turned off.
FX Loop Use
Many players feel that serious tone can only be had from a valve amp. FX processors concentrating mainly on effects such as chorus, reverb and delay (as opposed to distortion and amp simulation) may be combined with real amp distortion with excellent results. To avoid the nasty sounds usually produced by adding distortion after such effects, most guitar amps have an FX loop. This allows pedals or rack units to be connected between the preamp and power amp sections of the amplifier.