Simon, IMO it's important to know who you are buying tubes from. Many of the NOS tubes for sale are 30-50 years old and their history/storage/handling is critical. I have a vendor whom I trust because all of the tubes are tested and guaranteed. That being said, I probably have paid a few (actually more than a few) dollars more than I could have but peace of mind is worth it to me. You know the old penny wise dollar foolish thing. I don't need the repair bills and/or down time because I saved $10 on a pair of tubes.
Two things I would do are ask Mick about the specific tube you want to use and then find a reputable vendor who will back his product.
Two things I would do are ask Mick about the specific tube you want to use and then find a reputable vendor who will back his product.