If you buy a tube and it doesn't make a sonic difference, at least you'll have a backup for when you need it.
I notice the difference when I use new old stock tubes. At times it is subtle, other times more dramatic. It depends on the amp. A low noise tube can help in some situations as well.
The problem with NOS tubes is that they can get very expensive. Used tubes are an option, especially when you want to try different makes. www.tctubes.com
is a good honest tube reseller. As you can see, the prices can get up there.