Hello and Welcome DAN . I can not say about the Diamond modulator in particular . Though I sure like the modulating headlamp on my 1500 . It does get you noticed . Mine was on the bike when I purchased and I have no idea the brand .
I've been using a Diamond Star for the past year, without any problems.
Note that for a GL1500, you'll have to get the extra harness, that enables you to use it on a Dual headlight.
Hint (Illegal BTW): Replace the Light Sensor with a on/off toggle switch... that will allow you to decide when you want the modulator to function... Otherwise, with the stock setup, the modulator will not work in several "legal" low light situations such as as sunrise, sunset, and flickering tree cover... eg All the times you most need to be seen!