If you are using a digital output for DVD's, the player should not have any measurable effect on the audio quality - it should simply be passing the digital bitstream to the receiver or processor, which then does the "heavy lifting" of actually decoding the Dolby Digital or DTS bitstream, processing the raw digital data (bass management, time delays, and any additional matrix surround processing that may be desired), and converting the signal to analog so the amp and speakers can do their jobs. Upgrading to an upconverting DVD player will improve the video (assuming you have an HDTV, and that your HDTV has a DVI or HDMI input), but it's not something that will help the audio side over what you already have. Replacing your Yamaha receiver is probably the next step if your system still feels a bit "thin" sounding.