I have a pair of metal base 6N8S with black plates but without holes. The ones with the hole plates are considered the best and most desirable, especially the ones marked "1578". The 1578 tubes supposedly sound a lot better than the other variants. The pair I have sound very good, on the level of a good Sylvania GTA but not quite as good as my Sylvania 5692s (military version 6SN7). There is some controversy about the 1578 marking, as there are hole plate tubes without the 1578 marking but are otherwise identical, it seems reasonable that the 1578 was the same tube, just hand selected for use in nuclear reactors and jets.
The only tubes worth big bucks are the 1578 tubes with holey black plates, all the other metal base version are worth considerably less, so don't get ripped off buying any other version... I just typed 1578 tube into ebay and there was only one listing selling the real deal, but they are $180/pair... The non-holey black plate tubes I have are nice tubes and worth trying out, but I wouldn't pay more than $80/pair for them, and they are pretty rare as most have grey plates. A normal/common grey plate metal base 6N8S isn't worth too much...
http://www.ebay.com/itm/6N8S-6SN7-tubes-Metal-base-1578-MELZ-QTY-2-/221230364222?pt=LH_DefaultDomain_0&hash=item33825b763e
