I agree with the posts above which say that good tubes will make an improvement, but, a better overall quality of the amp will make the superior quality of better tubes even more evident. To me, the biggest determinant of quality is the output transformer, a part which also happens to be the most expensive.
Any particular preference would depend on a variety of factors, such as personal preference, system matching, etc., so experimentation is the only reliable approach to finding the right tubes. It could be NOS, it could be new tubes, it could be "cheap" tubes that will float your boat. It is hard to generalize.
In my system, I generally favor older tubes to current production, but, there are some current production tubes that are very good too. For example,I like EML meshplate 2a3 triode output tubes, but, they seem to be reliable only in amps that run them gently. Small signal tubes have even more distinct personality in my amps; I just happen to favor Tungsol roundplate 6SN7s and Telefunken ECC803S (apparently others like these too, as they are very pricey).
Any particular preference would depend on a variety of factors, such as personal preference, system matching, etc., so experimentation is the only reliable approach to finding the right tubes. It could be NOS, it could be new tubes, it could be "cheap" tubes that will float your boat. It is hard to generalize.
In my system, I generally favor older tubes to current production, but, there are some current production tubes that are very good too. For example,I like EML meshplate 2a3 triode output tubes, but, they seem to be reliable only in amps that run them gently. Small signal tubes have even more distinct personality in my amps; I just happen to favor Tungsol roundplate 6SN7s and Telefunken ECC803S (apparently others like these too, as they are very pricey).