Robert, I too have a 5555N11 radio and a KL503HD amp. I am using a Maco V58 with low SWR. Power supply is a Powermax PM4-120 and 4 gauge cables going from the PSU to the amp so it should be getting plenty of juice.
I do get the expected power levels from the amp itself when driving it with lower power, it's just the quality of the modulation goes to hell. The radio was purchase from and tuned by Scott's so I highly doubt that is the source of the trouble.
I am having a hard time hitting the sweet spot with dead key, modulation level, mic, etc. I'm told my modulation is very choppy and very nearly indecipherable. I've used the stock mic, D104 silver eagle and currently a Zatagi MB+5. I've been trying modulation levels all the way from 20 to 36 on the radio and about 3 to full blast on the Zatagi with minimal difference.
Are the settings you mentioned on your combo still working well for you? What modulation level (0-36) are you using on the radio?