Hi, firstly I'd like to say, do take care of the MP1 unit, it should be quite aged already. If you funk around wif it too much, it may not be able to take it.
Ok, what you are doing is pretty ok. Quite the norm. You are using your (Mesa Rect rite?) Recto head preamp to shape & drive the guitar signal. By using the send and return to plug to the in/out of the MP1, you are using the Recto head preamp to drive the input of the MP1.
And the output of MP1 to drive the power amp stage via the return of the Recto.
Just remember this, always turn all the gain & volume to zero first, before plugging in/out and adjust gingerly upwards the controls till you get your desired sound. This is to prevent going over the overload mark and damaging some parts.