Okay, for a Supra with the MB head, you MAY want to re-tune after turning the LOUD function off. Let me explain, but first a question: Do you hear a VERY large difference in stero volume between LOUD on and LOUD off, or is it just a slight tone change? I think it is probably WAY louder with LOUD on...
Okay, the explanation:
MB Quart is the one that I was thinking about when I suggested it was way more than just an EQ boost. There is also like 9-12 dB of gain added to the radio's output across all frequencies..... ON TOP of the typical loudness contour.
It is kind of a double edge sword; you are introducing distortion via the boosted EQ frequencies, which is bad, BUT the radio by virtue of the LOUD function's delivers more voltage with the LOUD function on. More voltage is good, as it allows amplifier input gains to be lower.
I'm not sure what year your boat is, but I personally tuned and documented the setup for the stock Kicker systems in the Supra plant for several years, (don't flame me for OEM product selection, I went with the most we could convince them to buy...
) . Had we gone with the typical proper system setup and tuning at the Supra plant with the LOUD function off, when one of you guys went and found the LOUD control and turned it on, there would have been instant amplifier clipping, as we would have set the amplifier input gains higher to achieve unity gain in the system.
For your boat, you need to decide what is important to you. Ultimately the total loudness of the music
will be better if you turn LOUD off and reset the gains using several of the gain tutorials I and others have posted. BUT..... And his is a biggie.... With LOUD off your amp gains WILL BE HIGHER
to get the amps to ful power when the radio is at full power. this has the potential to open up issues with amp hiss, clicks, pops and other nefarious, (bad) noises common with gains set too high.
This very instance probably points out the exception that exists with every rule. It is my opinion that the increased voltage that I think probably exists in your radio with the LOUD switch on may in fact outweigh the fact that you are introducing distortion via tone control with the LOUD switch on...
If it were my boat, I would first document the settings on the amps, verify that even with LOUD on that the Bass, Mid, and Treble controls were set to zero, then give it a good listen, trying to make several OBJECTIVE notes trying to put into words what I heard. Turn the radio way up and listen for noises, motor on and motor off. Note those as well.
Now go and turn LOUD off and reset things to as close to what you like as possible, using the gain tutorials posted. tone controls flat, all that good stuff. As mentioned, the gains WILL be higher. Now listen to the stereo and try to make comparisons which you prefer. It will be tough, but you need to be sure what you hear is real. Now turn the stereo up with no music playing and flip some switches, run the motor, listen for other bad noises that show up due to the increased gains.
If you do not introduce bad noises, consider leaving the stereo re-tuned with LOUD off and leave it that way. Just make sure your buddies NEVER go in and turn it back on...
You will find your stereo gets loud way too quickly, has no range of control, and clips extremely easily.
Sorry for the very long answer, but you need to know what and why, so you understnad the decisions made setting up your boat in the factory, based on the radio.