GW, welcome. You haven't described what sort of "difference" you'd be looking for, but if it's better sound quality from CDs, there's no realistic prospect of getting it from a different player. Optical disc readers have been a mature technology, achieving an audibly flawless performance at low cost, for many years. If you've read stuff elsewhere that players such as your S657, which also play DVDs, aren't "optimized" for playing CDs and a "dedicated" CD player is needed, this is pure hogwash. These players have dual lasers which operate at the 780nm wavelength which is optimal for reading CDs and the 650nm wavelength optimal for reading DVDs, DVD-As and SACDs.
If your player is still in good operating condition no audible "upgrade" is available. If it isn't, or you want some new feature which you haven't specifically described, I'd suggest one of the megachangers such as I use or at least a five or six disc player for greater convenience.
