You should give some NOS 5751 tubes a whirl, which are often used as a 12AX7 substitute. I'm using them in my Norh CD-1 and have had great results - more relaxed sound, less high frequency grain, better detail. These effects are subtle, but definitely there. Check out Joe's Tube Lore on the Audio Asylum FAQ section for more info and specific tube recommendations.
Beez
Beez