[EC900]

Not sure what wattage ratings are on the original radio (20w?) or door speakers (prob 10-20w range 4 ohm?) but your new amp increased output that OEM speakers could manage - for any optimum headbanger quality sound.

Yes, 48yr old speakers cones are not up to snuff anymore, likely the paper is too weak or broken and brittle. Upgrade needed. Dont replace w/o knowing speaker depth and fitment. Modern speakers will work wonders as will a modern radio with Bluetooth.
Also true, speaker wires play a role in quality if +/- are on connected correctly and gauged accordingly.

When I repaired my ‘77 door cards and original speaker grilles and found the speakers (2) were Pioneers to mate with the existing Pioneer stereo. That’s important - a matched system, but a hacked job.
One thing that helped improve and get better sound (on 2 speaker system) was replacing the missing speaker baffles (rain guards) and the plastic sheeting…if yours are missing. These direct sound outward and not into the hollow door cavity.
I have replaced flashy blue lit Pioneer a more mundane appearing Continental 7412 perfect for my needs, keeping just 2 door speakers older 50w 2 way pioneers.

Word of caution … if you’re not aware and going
to look inside the doors…don’t try to pull off the original speaker grilles with door card in place, they have one-way speed nuts installed on the backside to prevent them from coming out. You will break posts. I had repaired my OEM grilles that were haphazardly screwed back onto the door cards.