I've replaced the stock chinese tubes in the Jolida JD-603 with Matched Tesla/JJ 12AX7/ECC83's from Triode Electronics. After a few hours of run time, the sound is almost identical to my stock Marantz CD67SE in terms of frequency reproduction, but it is superior in terms of soundstaging. The music floats in the air much more than my stock Marantz, and instrument separation is heard much easier. I haven't tried using 12AU7, but apparently they can be used. You just get less gain (volume) using the 12AU7s.
