Well the Asgard 3 does not have a metallic sound to my ears. It's actually fairly full and warm. The Valhalla II is very good in it's own way. I've swapped numerous tubes with that one. Right now I'm using the Brimar CV455 from England. Really good tubes.
Are you referring to this tube? https://www.amazon.com/Matched-Psvane-UK-6SN7-Vacuum-Tubes/dp/B00N9T5ZHW This is sold as a pair though but I guess it wouldn't hurt to have a backup.