TELEFUNKEN, SIEMENS, VALVO, LORENZ, and other German made NOS: These tubes are usually characterized by an impressive open "air" at the top end. The soundstage is large, even in mono applications these tubes have a great 3-D image. The midrange is ruler flat, and the bass is tight and accurate. These tubes have a fine sense of dynamics, and most are impressively quiet. These are not "warm" tubes, and to some ears their lack of midrange warmth may be heard as bright. I tend to think of them as accurate, and their clean, focused sonic image is astonishing.
AMPEREX, PHILIPS, MAZDA and other Holland/France/Belgium made NOS: These tubes are a great balance of a clean, airy top end, nice midrange warmth, and accurate bass. They are very pleasant, clean, and musical to listen to in hi-fi applications. The white label Amperex PQ type or USN-CEP (same tube, the USN was made for the military) made in the USA are considered one of the best 7308 or 6922 types of all ever made. The D-getter and pinch waist rare types are also highly regarded, and are very quiet as well. The orange label types run a close second.
MULLARD, GENALEX, BRIMAR, and other British made NOS: Like a warm British jacket of the finest tweed, these glorious tubes have an attractive sweet warmth in their midrange and lower regions. The top end is silky and pleasant, without being rolled-off. The best of these tubes retain a fine sense of "air" at the top, and the upper midrange is smooth and liquid. These tubes reproduce the human voice, especially female voices, with haunting realism. The 1970s Mullard made have an attractive sparkle at the top with the rich bass, and these tubes are usually priced less than the older types.
RCA, RAYTHEON, GE, SYLVANIA, and other USA made NOS: This group is very diverse. The older RCA, GE, and some other brands of 6922, 7308, and 6DJ8 were often made by Siemens, Mullard, or Amperex, and usually are a bargain. The USA made 1960s vintage 6922 and 7308 are nearly always made for the military, although there were some early industrial versions made by Westinghouse and other brands. They are fairly good and usually priced very low. Their sonics are reasonably well balanced. The 6DJ8 tubes made in the USA, and the 6922 or 7308 USA tubes made AFTER 1975 are generally best used in test equipment that calls for them, as they are not particularly attractive audio tubes. The earlier 1960s versions are somewhat better, including the greyglass GE. Their prices are usually very low, putting them on par with the Russian or Chinese types. You may want to try them if on a strict budget, for they will surely outlast several sets of current production tubes, and probably still sound better than the Russian-Chinese trash. Overall, the USA made tubes are a nice surprise with their low prices as compared to the European types.
