2 Head units down the drain

superbase

So I have sunk $1000 in labor costs trying to get 2 different head units from 2 different shops to work in my 13 nismo with Bose system and both have utterly failed. The #1 problem is over-amplification of the audio and steering wheel controls. For starters, I can only crank the volume up to about 10% at which point the speakers become so ridiculously loud it hurts your ears. You can also hear a kind of "hissing" sound when songs fade out or if you pause them, the kind of sound you would expect to hear if you turned your stereo volume up to 100% without playing anything. And of course total failure of the steering wheel controls.

Whats going on here? Why cant anyone get a head unit to work in this thing? I dont know squat about car stereo's, so I cant understand all their technical jargon while they make excuses about what went wrong. They keep talking about hard wiring something through the bose amp, whatever that means. Are they supposed to bypass the bose amp entirely and just splice together the speaker wires with RCA cables and plug directly into the back of the head unit? Is the HU itself an amp? Or are they supposed to tie the new HU into the existing Bose amp in some unique way to prevent what sounds like double amplification?

Head units connected have been

Pioneer 270BT and Joying Android Head unit. Both sounded exactly the same.

At this point I'm just going to have them put the stock HU back in and I'll run a dang cable to the AUX in port, this is ridiculous.