And I got the answer
Hello Alex,
Good question - thank you!
All 4 inputs are separately buffered and then sum together.
You can use any or all at the same time / any time.
Just FYI: The 2 RCA inputs are buffered, then summed then go through the so-called ground lift circuit.
The 2 x 3-pin / combo 1/4" inputs are buffered then summed and then meet up with the RCA circuit.
This overall mono signal goes thrugh the rest of the circuitry.
(The ground lift switch ONLY affects the RCA connectors.)