The first GV250 I owned had what I thought was a bad muffler: loud and with baffles that rattled when I shook it or turned it end to end, so I replaced it with a good one and guess what? It still was as loud as the original muffler. Since then I've acquired two more GV250's and they were both the same: louder than I'd like. Since then I have lost about half my hearing so I guess getting old has its benefits...both bikes got quieter without my messing with them.
Maybe earplugs are your answer?