In relation to the meter the OP posted about, i just couldn't resist getting one to play around with, which meant i had to put it into an enclosure, and then a bit of tube rolling had to be done in the name of science
Just using the stock microphone it works very well, and i found it useful to know when people are shouting up the stairs at me when in my own little world with the headphones on. The line in i'm not so keen on as one channel gets tied to ground and the other feeds into an IC so i dont bother using it. With speakers it works great just using its inbuilt Mic.
Pics are the stock 6E2, Mullard EM84 which is basically the same tube but better made, and a Telefunken EM800. The EM800 is my favorite as its a single bar from bottom to top vs the two bars that meet in the middle. It is ~€14 for the tube which is more than the whole meter cost in the first place, but hey its cool so worth it. The other types of tubes are only a few quid each, and there are at least half a dozen different styles that will work in it.
I'll give it a 10/10 for novelty value, but 4/10 for actual usefulness.