I believe they are 4" or 4 1/4". Only personally seen two GL1500s with amps, one was fitted underneath the left faring pocket and the other was behind the floppy part of the pillion seat area, wrapped in plastic to keep the rain out.
And what is the verdict on the Polk replacement speakers? I know that Honda could put something better in their bikes but I think they have a deal with aftermarketers right? Just like everything else huh!................ Ride On
Those Polks sound like a god alternate. They are marine grade speakers and also for bikes. I've got a home system with Polks and my computer speakers are Polks. Very satisfied and will look at these for the bike.
The Polk DB-501 speakers are a nice replacement for the old originals. Much clearer and more available volume so obviously they are a decent match for the factory amp.
I did have to elongate the screw holes on the bike a bit since the new speakersare slightly smaller but otherwise they are a drop-in replacement. Being "marine-grade" means they should hold up well under any weather they get exposed to.
I got mine from an eBay seller and I had no problems with the order.