All of the classic WWII/Korea era tubes sound great. These include those designated VT-231, JAN (joint army navy), the GTA and GTB series. There are various technical differences between the tubes but they all work in the AE-3.
I have bought (almost exclusively here on the Gon) and enjoyed many of the classics for my AE-3including Sylvanias in many incarnations-especially the so called Bad Boys, the TungSols, the RCAs with the grey glass, the RCA Redbase 5692s, and the Raytheons all have something a little bit different to offer. To state the obvious, this is all about synergy - what you really are looking for is the tube that makes the rest of your rig sing. (more tipped up or less, faster or slower, tighter bottom or not etc)
In my experience, the differences range from quite subtle then the extreme. In terms of what you will hear, 6SN7 aficionados distinguish between tubes by their air, lushness (fat), how much sparkle their is at the high end, how tight the bass is. While no one has a word for it, some are definitely faster and have more attack then others. Too much of this and people talk about the tubes being etched.
Some tubes tend to be quite microphonic and are obviously undesirable - this is something to discuss with the supplier before you buy. BTW this cannot be tested by thwacking the tube with your fingernail... Also the concept of matched is not about how the tubes test for life expectancy so much as it is how close the two triode sections test to one another within the tube, and then how well the tubes compare. Rare to find anyone doing this.
FWIW I have played around with various brass dampers, as well as the Pearl Coolers and the Herbies. Tends to be a bit tube dependent - sometimes they help, sometimes they kill some of the air.
I did find that cables make a big difference - a fairly neutral (but not critical)presentation is key if you really want to dig into the nuances. And I found that my AE-3 definitely benefited from power cords, footers and a fat wood block.
BTW, the Valvo (an English brand) is most likely a relabeled Mullard, the European designation was EC33.
There are a number of websites devoted to the tube and the differences you will enjoy:
Of course it is possible to get quite caught up either pursuing the ultimate sound or pursuing every flavor. I know this is not sporting to say, but IMHO once you get a good set I question the wisdom of continuing the chase - I will be posting about 20 of them soon. LOL
enjoy - its a great pre