An interesting thread at the avengers.tv international forum regarding the upcoming UK reissue of the complete series of The Avengers on DVD includes encouraging hints of improvements in sound, extra materials and (crucial for the earliest episodes) the potential addition of subtitles.
Of course, being American, I compare the existig R1 A&E/Canal DVDs to the broadcast quality of the prints I watched on television in the early 1980s—on that basis they look absolutely brilliant.