Having been a "pioneer" in modifying the GL1800 audio (click the pic above for more information) I have a few comments on this thread.
#1 - 2 ohm speakers will not harm anything. Most all audio amps are rated for power output at 2 ohms. This way, the manufacturer can boast of higher wattage ratings. Hook two four ohm speakers in parallel and you get two ohms. The amp will output twice the power to 2 ohm speakers than if you hooked a single 4 ohm speaker to it so the maker gets to say "our amp puts out 200 watts" rather than 100 watts. Fact.
The speakers in this thread are AUTO DOOR speakers. 5.25" speakers are designed to be mounted in the DOOR of an automobile. Certainly weather proof enough for application into the GL1800 speaker enclosures. I have been running four 4 ohm DOOR speakers in parallel on my wing for SEVEN years. I power wash, ride in the rain and thoroughly enjoy the audio. Fact.
#3. I have to hand it to EAGLEFAN67 for his patience with all the naysayers in this thread. Congratulations on your independent thinking. Go outside the box and they come out of the woodwork.
One final word: nearly NOBODY in this thread has the YEAR of their GL1800 in their signature or personal info under their name.
I think it should be a board rule that everyone must put the year of their bike in their signature! .Really helps us all.