Here's how Impediance works........the system on your bike was designed with a certain amount of resistance in mind. So many watts of output into a given amount of resistance. If you lower the amount of resistance in the speaker, it's kinda like opening the floodgates on a dam, won't hurt the dam (Amp) but could hurt the river (Speakers) so, putting a lower resistance speaker in your bike won't hurt the Amp but will put the speakers, themselves at risk.
Of course putting a Higher resistance speaker in is like closing the floodgates on the dam, it'll back up the water and could damge the Dam (Amp) cuz there ain't no place for the power ta go.
I think the HK system is rated at 40watts into 4ohm. The 2ohm speakers will work and should be louder, but will probobly burn out faster. I would hesitate to put a Higher Resistance speaker in but lower is ok.
By the way, I put the Biketronics 6" speaker system on my Roadglide, they are 2ohm and sound VERY, VERY good. Good luck.....Rock ON!