FX loops can differ a lot in different amps. Some loops are pedal level, some rack level, some can do both (e.g. most recent Marshalls). Then there are serial loops and parallel ones (that blend in the effect pedals and retain part of the original tone), which behave very different, too.
I found that usually what works best with amps is a good rack unit, where you can adjust the input AND output levels, so you can make sure that the amp gets just the right levels of undistorted signal into its power stage, this can be crucial for dynamics, clarity and punch.