Can excellent tubes transcend modest amps?

A pair of these: (http://www.tubedepot.com/pa-6sl7gt-3.html), and NOS TungSol JAN CTL/6SN7GT roundplates or Sylvania JAN/CHS/6sn7W tall bottle tubes from the 40's as drivers in the A-88T would probably satisfy your craving for a taste of the high-end pie(entry level, but solid). That would depend on your SPL needs, and the rest of the source material/system/room combination of course. Personally- I'd use Winged 'C' 6550's(the latest iteration from the St Petersburg factory) as output valves, as I know them to be a very clean tube. Slightly more power with the KT-88's, and you could well be pleased by the stock outputs. You didn't mention your tastes in music, or listening levels. Be certain to buy matched/tested/certified pre-amp grade(very low noise) 6SL7's for the piece. Finding NOS 6SN7 "Grails" that are screened for noise will be tougher, but they shouldn't be in as high a gain stage anyway.

Better tubes make a mediocre amp better but would go so far as transcending it. On the other hand, the are great amps made with so so tubes and still sound great. I would say great amp transcends modest tubes

there is no definitive answer to the question posed by kanuk. until the criterion of "better sound" is denoted, the best course of action is to listen to an assortment of tubes, where there are return privilegs , to determine ifsonic improvements can be attained.

there is a risk when buying another amp, just as there is a risk of dissatisfaction when changing tubes.

our hobby is based upon subjective evaluation, personal preference and a lack of predictability.

it is best to be an empiricist without preconceptions.
without the facility to listen to alternatives, one is left with speculation and risk.

I will add my voice to the chorus of answers above. The answer is NO! The sound from your amp is the sum of its parts. I think tube rolling has acquired prominence because it is the easiest part to tweak. After all, you're not going to be changing the output transformer in a hurry, are you?

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).