Speakers only work at low volume

I'm at a loss here, I bought an old truck with the head unit missing, so I bought a new one and hooked it up. The sound that came out sounded okay at about 20% volume, past that it sounded shakey and distorted. I assumed the speakers were the issue, so I bought new speakers and had the same problem. Next I ran all new wires from the head unit (constant, ignition, ground, and speaker wires) still did it. So I figured there wasnt enough power, so I got an amp and everything sounds distorted at all volumes with the amp hooked up. Idk what else to try and I need a radio I can listen to. Im not looking for the best sound system ever, just something to sound as good as the factory.

I really need help getting this fixed, shops quote me around 500 to fix it and that is not happening.

Head unit: Pioneer DEH-S1010UB Single CD Receiver with USB

Speakers: Scosche HD6904SD - 6x9" 4-Way Car Speakers (Pair)

amp: BOSS Audio R1002 - Riot 200 Watt, 2 Channel, 2/4 Ohm Stable Class A/B, Full Range, Car Amplifier

amp kit: BOSS Audio KIT2 8 Gauge Amplifier Installation Wiring Kit