Have you measured the draw of your sound system against the available amps your electrical system can provide? Have you load tested your battery to determine what level it is at in terms of available power, what amount it might be providing may be on the downswing. What other components are you powering with your system (light upgrades, high wattage phone chargers, etc.)? What amperage is your alternator rated for and have you verified output is still at rated capacity (they do fail)? What gauge and type of wire are you running from power supply to amp, and then out to speakers, and what did you tap into to run your amp? Have you tested your gain setting to minimize distortion? Have you checked every ground setting not just of your audio system but the whole truck? Same with power lead wires, have you checked and cleaned all connections?
Your drop at high RPMs tells a tale that there is a voltage problem, which can be related to low amps but also wires, etc. if you haven't run through all of the above, that is where you should start. Don't just start replacing things like the battery or alternator without a purpose because you may not actually fix the problem. Testing is critical, especially the battery. Could you need an higher output alternator, maybe. But not sure how long you have had your truck. How long it has been set up with your audio, and how long you have had the problem. If it wasn't an issue in day 1, something changed and you have to find it.
Let us know what you find.
