Yeah... Music Hall is a pretty well respected name. They are more known for their audiophile turntables. The upgrades are internal replacements of cheaper parts for more expensive ones that are directly responsible for the music you hear.How's that for a very technical answer. It's all too technical for me to explain. There is more information on the Music Hall and the upgrades here.
http://www.6moons.com/audioreviews/musichall/cd25.html
and here
http://www.underwoodhifi.com/mod_musichall.html
and here
http://www.musichallaudio.com/mmf_products.asp?show=false
Many here will dis this upgrade stuff. All I know is my CD player sounds incredible. Better than most any I have auditioned and I have a listened to many. They are generally well reviewed in press and web reviews.
oz
