I have a Boss ME70 in front of my G-DEC 3 Thirty, if you use a clean tone on the AMP the effect pedal can be used and the sound is good. You can also add the reverb and delays from the G-Dec into the Mix. It is overkill to use overdrive effect from of the amp if you are using overdrive pedals in front of it.

Also I found that using a pedal in front of the AMP is easier if you want to switch sounds and off during a song. If you use the amp with backing track it's harder to change setting during the song since the backing track start all over gain if you switch (even with the optional footswitch, this does not happen with the quick access modes but then you have only 4 options)

HTH,

Jurjen