OK, so I have two JBL Stadium GTO620 speakers in the front, which are 2 ohm.
I have Alpine SPS-610 speakers in the back, which are 4 ohm

I have a Pioneer DEH-x9600bhs head unit.

Using the four preamp RCA jacks on the head unit, I connected into an Alpine KTP-445u amplifier. It's the little Class D one.
Also using the two preamp RCA jacks on the head unit, I connected a Kenwood KSC-SW11 powered enclosed subwoofer.

It sounds absolutely wonderful, most of the time.

The problem I'm having is when the radio is loud, the sound on the four speakers will cut out at very high, loud notes. The odd thing is that the subwoofer is fine. It doesn't miss a beat. The four speakers will resume musically after the high note has passed, so no resetting, or repowering of anything is necessary.

1. The yellow power line that I use to power the Alpine KTP-445u amplifier comes from the back of the head unit.
2. I tapped into that wire to power the Kenwood KSC-SW11 subwoofer as well.
3. Neither the head unit, nor the amplifier, nor the subwoofer share a ground point. They all have their own.
4. The head unit is in charge of crossovers. I have a low crossover set at 63Hz. I think the high crossover is the same, 63Hz.
5. Head unit spec sheet never indicates that the radio can handle 2 ohm speakers, just 4 ohm.

Can anyone point me in the right direction?

Thanks.

Could the fact that the wire from the radio

 

To all following this thread.

I replaced the amplifier with a larger one. I bought the Alpine KTA-30FW. Still a Class D compact amp. It has solved the problem. It now goes louder than I need it to (Gain is at just below half) without any cutting out. If something changes, I will come back to update this thread.

I did run a fused 8AWG wire straight from the battery to a secondary fused splitter, where both the sub and the amp get direct power.