So, you believe it might be bad tubes?Seems like a reasonable bet to me. If it is a bad tube, it would figure to be either V5 or V6 (both are 12AU7's), since the other tubes are not involved when line-level sources (CD/DVD or cassette) are being used.
Since V4 is also a 12AU7, but only supports phono playback, you might try swapping it with V5 and V6 (one at a time). If either of those substitutions fixes the problem when one of the line-level sources is being used, you've found the bad tube!
Regards,
-- Al