I was fussing over my Pro Reverb today with a few choices in 6L6 at my disposal. I have a set of old RCA 6L6 that are original to the amp. Unfortunately, one is a little loose mechanically inside. And I have a couple pair of Sovtek (6L6GB and 6L6WXT+) and pair of Fender labeled 6L6B GTs. I've tried each of these sets in the amp, did a bias adjustment each time. I have to tell you the RCA's earn their reputation for simply being sweet sounding. The other ones just sucked being more or less excessively cold, spikey, sterile, and/or brittle in varying degrees depending upon the tube. The RCAs went back in. This got me thinking, though... Do the old tubes sound good in these old amps because that's what the designers had to work with when they designed these amps? Do a lot of new builds sound better with NOS tubes because the designs are based mostly upon the old amp designs? If a new amp design were developed around the characteristics of what are currently some "less desirable" tubes, would the new amp then sound great with this tube, and not as good with NOS tubes?