What's the difference between the Audio Research..

They claim electical, optical, and mechanical part changes. I upgraded from MKI to MKII and I thought it was an improvement - more clarity and articulation making it easier to understand lyrics and sense microdynamic shading. Looking inside, the one obvious change was more internal dampening material throughout the chassis.

Hi,
Keeping in mind that the player(s) will sound different in different systems and rooms etc.,I will share the following experiences with you.
I used to own the mk1 with all Audio Research gear and Vandersteen speakers w/ Audioquest cables. A common enough system match. I had the AR ls16,pre-amp and the 100.2 ss amp with the Vandy 2ce sig then with the3a sig. I also had the AR vs110 amp.Overall the cd3 mk1 was very dynamic, clean,detailed non-euphoric player. Also very accurate and at times could be a bit bright,but not to the degree of being harsh.It was the BEST player I had ever owned . At one point I had to sell it and it took a long time for me to part with it.So I then began to audition other players. Among them the Audiomeca Mephisto -too smooth ,no dynamics. The Xindak SACD, ok for the $ but ultimately uninvolving.The Audionote 3x (I believe) a very nice sounding player.Cal labs Alpha/Delta another great combo.and the Philips 1000 SACD,not impressive with SACD or cd.
1 year later and I change my system totally .I now own the Wright La12p pre-amp/wright mono8 set amps w/ Reference 3a monitors and VH audio cables(one of the biggest deals in cables today)
Everything sits on Mapleshade platforms and cones. I also have the AR cdmk2. This player is more transparent,detailed,still has that toe tapping dynamic quality with absolutly no glare. It's a much more refined player but keep in mind my system is also giving me a much more realized mid-range. I don't think you could go wrong with this player.For me it's the one piece of gear I'll never change.
BTW the Sim Moon Eclipse is ,to my ears ,close to the cd3 but not as refined.
Good Luck

Why would a cd player sound different in different rooms? Surely the sound may be different from one room to the next, but that has to do with the speaker/room interface. How would you assess that the problem is upstream in the system? Enlighten me, please.

A "bright" cd player may not sound so bright in a "dull" room.

Take notice of the frequent availabilty of used CD3's here on Agon. I am a former mk2 owner FWIW. If you like the sound of 1 bit converters, then this machine maybe for you.