Mesa Mark 4 and Simul Satellite 85 Issue

Hello. I have a 2008 Mesa Mark 4 B and Simul Satellites. I connect Satellite Send to Satellite
Return. When the jack in Mark 4 is connected to the right return (mono) The signal from Mark 4 is
ok, but from the Satellite the signal sounds bad (looks like a signal from a recording out). If
the Mark 4 jack is connected to the left return (stereo), the satellite sounds perfect and the
Mark 4 sounds bad.
This happens when the selector switch is on Fx Loop on Full time. At the FxLoop Switching
position, both amps sound good, but I have no effects. I control Mark 4 with Musicomlab mk3.
I tried removing the effects from Fx Loop and the guitar directly in Mark 4 with the same result.

There are a lot of options there on the back panel of the Mk 4 -- have you read the manual on some of the hook up choices? It's on page 10 here:
http://jdarks.com/files/Mesa_Boogie_Mark_IV_Manual.pdf

Why are you running the send out of the Satellite? Are you using the preamp in the Satellite or the Mark IV?

You should route your guitar like this: guitar to Mark IV input. Mark IV send to stereo fx unit. FX out (one side) goes to Mark IV return, the other side goes to the Satellite return. You should not have anything connected to Satellite input or send.

That sounds right to me, this will hook you directly to the power section of the Satellite, and give you a true extension of the Mark IV sound.