Stan; Thank you for sharing your results for those of us who have SINAD meters but cannot afford a noise figure meter. I also thought your statement:
"An additional test was performed, adding a fixed 10 dB BNC attenuator after the preamplifier and before the RX. The SINAD measurement did not change. This indicated that at least 10 dB of excess gain was present and can be eliminated. It is always best to minimize system gains to maximize the ability to handle strong signals and to reduce higher order intermodulation distortion."
is very appropriate for radio system designers. I work in the land mobile radio field and many of the trunked systems use tower mounted preamplifiers with excess gain. The use of an attenuator between the preamplifier and the multicoupler to control IMD is often overlooked, misunderstood, and as you demonstrated may have a tremendous impact on the performance of a radio system.
Thanks for sharing this.