I know this is a somewhat common mod (if I can use that term with a not common amp), but what is your purpose? I know 8417 tubes are not common, but they are available and not outrageously expensive (like 6bk11s). I love the sound of my 8417 Thunderbass and feel it's well worth the trouble to keep it as spec'd.
Of course, anyone is free to mod their own equipment as they see fit. I just think it's become a go to fix that doesn't need to be.
8417ers unite!