I have a 2005 Mazda 6s Hatchback with factory Bose audio system (2 ohm).

I replaced the factory speakers due to one being blown and not being impressed with the overall quality of the sound. After I put in the speakers (Boston Acoustics SC60 6.5" component set up front and SE55s 5.25s in the rears, 4 ohm) I found that the stock Bose amp had a built in EQ that had the mid-bass levels cranked to overcome the limits of their paper cone speakers. I replaced the stock amp with an Alpine MRP-F300 (4 ohm). I cut all the wires from the stock amp harness and spliced what were supposed to be good RCA cables to the signal lines and plugged them into the amp. I used the factory internal speaker wiring to run to the new speakers. After all that, now I have almost no bass at all and have to turn the volume up to nearly the max to get much more than moderate volume.

The local shop said that the problem was due to the signal level coming from the stock headunit being low. They tested the rest of the wiring using a different input source (the installers smartphone) and the guy said that the speakers sounded great and he had to turn the source down because it was uncomfortably loud, so I know the speaker wiring is good.

The shop tried to sell me a replacement head unit or a line driver. I'm highly suspicious because they sounded like they were either looking for an excuse or trying to sell me something else.

2 questions...

1) Since the volume was fine with the Bose amp in place, could the splice to the RCA wires have caused signal loss? The RCAs were heavier gauge than the source wires and came with a decent amp install kit, so I have to assume that they are in good shape. The only other explanation I can think of (since many Mazda 6 drivers are using the good quality factory head units with aftermarket amps/speakers) would be some kind of damage caused by the amp install or a mismatch between a 2 ohm signal to a 4 ohm amp. (forgive me if that's an incorrect statement, I don't really understand the ohm ratings outside of basic resistance, or if a signal would have an ohm rating at all)

2) Would going to a smaller RCA wire change anything? The stock signal lines are a significantly smaller gauge than the RCA lines I spliced them to. Could going to a larger wire account for any of the loss?

If anyone has any suggestions, I'd love to see them!

I wanted to add a quick note. The car has an in-dash 6 disk changer and steering wheel mounted controls. I love having the controls on the wheel and am not a fan of the look of aftermarket head units in this car because there is a conversion kit that I'd have to get. The kit still leaves a lot of empty space and does not come in the same color as the rest of the dash. Having it be a different color would change the look of the car more than I'd like.

Spader, I already had the car in the local shop and their only suggestion was to replace the head unit or a line driver.