Question from tube y newbie

Equally biased tubes i.e. matched tubes certainly produce less distortions especially on the extreme passages of music.

So would it be safe to say that, as long as I can bring all the tubes into correct bias, a new single output tube should be just as good as a matched pair or quad?
The age difference is still an issue, however.

If your amp allows individual biasing of each output tube and does not 'require' matched tubes, it's perfectly OK to replace just one. However, as Marakanetz said, overall distortion will be lower with a matched set. Whether that difference in distortion levels is noticeable by you is another question.