I'm not totally sure why bose came into this, but they certainly have the expertise to make fantastic sounding systems. I agree that they don't necessarily actually do that in many cases. But they've made a ton of money off of marketing it. I'd guess a Porsche 911 with the Bose system sounds pretty good. They seem to target the mid-range systems in most vehicles. Probably a lot like the jbl system in the Toyota/Lexus lineup. It's not expensive and sounds okay. The Lexus ML systems are usually a pretty good value. Rarely are they over about $2500. And in the used market the cost is typically zero to find one with vs without. Plus if you want a GX with crawl control - it only comes in the Luxury model for most years, so it's almost always going to be together with crawl control.
I certainly don't dispute that a 4Runner could be made to sound better than a GX with the ML option given unlimited money and time - but I don't think you'll get there for $10k. Certainly not if that price has to include labor of stripping out the entire interior, rewiring, custom making new locations for a dozen more speakers, replacing the alternator, etc. All of these are quite easy to do at the design stage and a lot more cost effective for the manufacturer. And I have no idea what you'd do to smooth out the v6 to make it comparable to the v8 when you're actually moving. It's not exactly "silky smooth". So you could probably add some very low noise tires and keep the full OEM suspension system with all rubber bushings for the softest possible ride with minimal road noise.
But then, why not just buy one that has it all done already? That's why the logical choice points to the GX. Even if you're going to upgrade it, the GX has a better starting point. It already has a 17 speaker, 12ch, 7.1 audio system. Starting there seems like a much easier path.