Ahh... a third party accessories provider ... I haven't seen too many post about these systems and or post regarding the sound system overall.
One forum member looked at swapping out the rear sub speaker within the rear case; however, the larger magnet required cutting up the rear side case and eating up much needed case real estate..
I love streaming music while I'm riding, it helps to past the time, especially on long road trips... I'm pretty practical about the fidelity that I'm going to get while riding my bike; the bar is set pretty low considering wind at 70+ mph, not in an enclosed space and overall real estate to install hi-fidelity components..
The Marshal system is one of my favorite attributes about the Model B.