Mrtennis,
you did not mention the preamp you are using, I assume tubes. Here is where I would start with the Mullard NOS tubes first. The Mullards will roll of the highs slightly. bass will be a little loose. Midrange lush and sweet.
Up until about three years ago I was using an Audio Research LS3 SS preamp with a Audio Research VT50 tube amp. I have hundreds of early CDs (late 80s early 90s) that just set in the rack. Sound quality was either harsh or bright sounding. At a moderate volume level listener fatique would come quicky after just listening to a couple of tracks. The ARC LS3 is a great SS preamp, problem is it will pass the source material straight through without adding anything or taking anything away. In other words, good in good out... Bad in bad out....
I tried different ics. Tried ferrite beads on the power cords of the CDP and preamp. I even tried an equalizer, didn't like that either. So I just quit listening to badly sounding CDs....
Then about three years ago my Son brought over a Sonic Frontiers SFL-1 Hybrid preamp for me to listen to. The unit uses a single 12AT7 tube. Two tubes came with the unit one Sovtek and one NOS Mullard. With the Sovtek installed the sound from my system, the preamp pretty much sounded like a SS preamp. Good CDs still sounded good. Bad CDs still sounded bad to me. Pulled the Sovtek and installed the Mullard tube. Right out of the gate I could hear a big difference. Highs were rolled off a tag, bass not quite as tight, midrange, warm, lush, wet. Old CDs, well, they did not sound great but listenable, not bad at all.
About two years ago I move onto a Sonic Frontiers Line One preamp. I have never tried Mullard tubes in the Line One. I do have a few pairs of the early 60s NOS Siemens CCa tubes, great tube but very pricey today. Also a few NOS pairs of Amperex USA PQ white label 6922 tubes, also a great sounding tube. At the present time I am running the PQs in my preamp.
As for the ARC VT50 I am using all EH tubes.
Here is a link to check out,
http://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.htmlJim