As shock this mentions, we both have a 2018 Supra SL and are on the same lake. We both have the same problem, that our boats pop loudly on startup, but only if we are in battery position 1 or 2. It does not happen if we start in position 1+2. In the SL there is no way to turn off the stereo completely (there is no on/off button on the touchscreen), only turn it down to volume 0 or mute, but it still pops in this situation. It would be nice to be able to turn off the stereo completely, which is why we were looking into wiring a switch into the system. Alternatively I suppose you could start in 1+2, then switch to either 1 or 2 when stopped, but this leads to a bunch of flipping of a battery switch when out riding. The other thing we have looked into is ditching the stock 1,2 and 1+2 perko for a blue seas switch and ACR (7650). Apparently Supra is aware of the issue, but they have not come up with a fix yet either. All they have said is that when starting on 1 or 2, the starter pulls enough voltage to cause the subs to turn off and on, causing a pop. Not sure why this would happen if starter and house batteries are isolated.