Just finished an install on a Road King. Fronts limited to 6.5" coax in lowers only, unless there is an option to front mount a horn I'm unaware of on a bike w/o upper fairing?

Rears are PRV 8" with ST25's in Twisted 8 lids
JL 600/6
Wetsounds BT receiver

We attempted to use Hertz HCX 6.5's leftover from his previous setup, but discovered tweeters are blown. I had my doubts anyway with only 92db sensitivity that there would be any front stage.

I know DB WDX6Moto's are very efficient, but wanted to see what other options are there for 6.5" coaxials that are efficient enough to keep up with pro audio in rears?

 

With the right grills you can mount horns on the grills. Although expensive XBS has some killer ones.

 

American Hard Bags has a kit to install the HertZ ST25's in a chrome cup that mounts on the bars, very cool looking but pricey. if you can find a way to set up horns I'd use pa's in the lowers, The PRV's have a pretty broad response for a PA speaker, you could try the ARC 602's if you have enough power to send to them.
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U can run another 8 and horn...dirtybird makes lowers that house and 8 and a horn...just run another prv and horn up front...run all horns on 2 channels...and each set of prv will get 2 channels...that would be rockin!!!