Our customer feedback:

1.  I leave mine plugged in all the time. It works perfectly. There is zero noise added. The box is all metal and completely shields the connections inside the box. I see some reviews claiming it's noisy. The box is completely passive, no active components inside, no transistors or IC's or anything. Just wires connecting the switches and jacks and completely surrounded by the metal box. So it's not possible for it to add noise.

2.  I think the mistake some people make is to connect the two (A & B) amps to different electrical outlets--which creates a "ground loop" and can result in a lot of hum. Don't do that! Plug your amps into the same outlet box to eliminate ground loops and it will work fine. This is just common sense when working with two amps.

3.  I am an electrical engineer and have designed audio equipment and understand the principles involved. Ground loops create noise whenever you do not observe these simple precautions. If you have one amp plugged in on one side of the room, and the second plugged in on the opposite side of the room, and connect them both to the A/B box, you will most likely have a lot of "noise" (60 Hz hum actually). That is not the fault of the box, it's the consequence of the ground loop currents in the cables connected to the box. The voltage produced by the ground loop currents produces a voltage in the cables that gets amplified along with the guitar signal.

4. I bought mine, instead of building one myself, because the cost of the product was less than if I were to buy all the switches and jacks, and the box, and make one myself!