I think the Mullards give a warmer, richer, smoother more full bodied sound, better high end extension without feeling like an icepicks being jammed in your ear, midrange to die for , can you say Phat (thats what I like about Mullards the most probably due to type of music I listen to), tight bass, soundstage is a huge improvement; wider and deeper...the stock tubes seemed thin to me, lifeless, without character and the trebles had limited treble extension and when pushed were almost ear piercing and the bass was flubby...I've read the cheap fix is to replace with some RCA 6201s as they tend to not make the sound anot so in your face, as the 6201 is a 12AT7 tube it has like 30% less gain compared to the factory installed 12AX7, the less gain of the 6201 helps smooth things out so that the soundstage increases so you can hear the whole mix, 5751s will also work for same reason as they have less gain...same guy told me to put a phone book on too of the player (nice look huh?) to reduce vibration instead of using more expensive tube damping devices, it essentially will effectively damp tube vibrations that can cause ringing, microphonics, and sonic distortions. i use brass eights i bought inexpensively on Ebay but he has a cheap fix...hope this helps, lemme know
here are comments from the owner of thetubeshop.com
RCA Black Plate Double mica 5751. Try these in place of the 12AX7 in
your tube CD player. They sound richer and more detailed
Try RCA blackplate 12AT7 in your tube CD player in place of 12AX7s
The lower gain really opens up the sound. A customer turned me on to this
and I loved the diffrence in sound.