It depends on if you plan to leave local control on in all your synths and have the “MIDI ECHO” set to STOP ECHO in the Pyramid. This is how I have it set in my setup. But if you have it set this way the MIDI out of the Keystep will not pass “thru” the Pyramid so that it plays any synths. That is why I have the additional mergers so that the Keystep goes both into the Pyramid and also to the synths.
Now the other way to do it, and how Mr.TUNA does it, is to turn off the “local control” in all your synths and then have “MIDI ECHO” set to ALLOW ECHO in the Pyramid. In this arrangement the MIDI out from the Keystep will pass “thru” the Pyramid to the synths but so will all the MIDI information coming out of all your synths. This will create a MIDI loop so that is why you have to have “local control” set to off on all your synths.
If you are OK with and are able to turn off “local control” in all your synths then it’s OK to do it the simpler way with just one merge box and one thru box. But doing it this way means you will always have to have the Pyramid powered up so that the MIDI signal can go out of the synth, thru the Pyramid, and back into the synth to play the sound. Remember “local control” will be off on your synths so playing the keyboard will not play the synth directly.
But you did mention that you want to be able to play the keyboard of the matriarch and p6, so local off is not really an option. In that case you will need the extra thru and merge box. The thru to split the signal coming out of the Keystep so you can send it both to the Pyramid and then back through the second merge box back to the synths along with the pyramid MIDI out.
It’s up to you on how you want your synths to behave. Controlled directly or controlled with MIDI first needing to pass “thru” the Pyramid.
To simplify things for you If you want to leave “local control” ON in your synths and to prevent any MIDI loops you will need to have the “MIDI ECHO” set to STOP ECHO in the Pyramid. But this will also prevent your Keystep from sending MIDI thru the Pyramid out to your synths. Adding the extra THRU and MERGE boxes solves this problem.
Another way to get around this problem is to go in and set the “MIDI ECHO” to ALLOW ECHO in the Pyramid every time you want to use the Keystep. This will allow the MIDI information from the Keystep to pass thru the Pyramid to your synths. A bit of a hassle but is doable and you wouldn’t need to purchase anything extra. But you will have to remember to go back and set the “MIDI ECHO” to STOP ECHO in the Pyramid everytime you want to start using your synths again.
Hope this helps!!