Hello Karl,
The purpose of the diode mod is solely to protect the IC402 (MUX chip) from some unwanted parasitic ringing which may occur on some bands while transmitting (QMX in TX mode). While in RX mode, the diode has no effect, being reverse-biased. As far as I can tell, It won't eliminate any birdies, nor will it alleviate any low-power output issues.
The tones that you describe, that go up and down the band, I can hear them as well, on both the QMX and IC7300 - they're not caused by the kit. However, the fixed, very narrow birdies may be caused by the SMPS modules. Hans knows about them and will fix that in some future firmware update.
Regards,
YO3GFH
op. Adrian
