You will definitely notice the difference in sound putting the m13 before your amp's input versus going through your amp's FX loop.

and unless you need 8 simultaneous effects, you don't need two m13's to acheive what you want. The M13 has an effects loop built in, so you can have 2 effects going to the input of your amp and 2 effects running through your effects loop.

That's how I use mine live, and it's definitely a noticable improvement putting your time based effects (like delay) in the FX loop of your amp.

A great way to hear the difference is to setup the M13 using the 4 cable method, and set up two FX switches on the M13 to have the exact same delay setting. If one is going to the input of your amp and the other goes through the FX loop, then you can A/B back and forth to hear the difference in real time.

That's what sold me on using 4 cables instead of 2.

I just wish they would make the M9 mono with an FX loop (instead of stereo) so I could have the smaller footprint and still have my delay go through the FX loop.