I upgraded to an XM radio subscription on my Garmin. Input through the existing system is perfect. I replaced the front speakers with new J&M speakers two years agao, and found a drop in replacement Kenwood weather restistent model speaker for the back - what a difference!
IMO you will never do enough to compete with the system on a GL1800, but you can get good sound and hear it at over 70 mph if you replace the old speakers and make sure the connections under the seat and side panels are free of corrosion.
This link might show the pic I posted -
http://www.valkyrieforum.com/bbs/index.php/topic,20745.msg179837.html#msg179837