Sounds like an ohm issue. When you had all four speakers hooked up they were in series. That set up will double the ohm rating and cut the sound output in half. By removing one set of those speakers you have restored the required ohm rating and it sounds fine. I would bet that the factory speakers are 2 ohm and the aftermarkets are 4 ohm. If you were to put 2 ohm speakers in it and reconnect the factory ones I would bet they will all work fine

Don