Hi,
I have fitted myself the two tube upgrade kit to my old CD7 and did not find it deteriorated the sound - truth is it did not change, compared to a good set of matched 6h30.
Later a close friend also had it fitted to his player by the distributor and considered it an improvement. As he was listening to an unit with defective 6h30s in the power supply immediately before it is not a good case.
Owners using RCA units immediately noticed the hum noise when the tubes were not matched, people using the XLR output only noticed a much poorer sound.
My CD8 is still burning in ...