do new tubes, such as EL34's, need time to burn-in before they sound good?
Yes.
I replaced the stock EL34's in my Jolida 302B with JJ Electronics EL34's. The JJ's sound a bit thin as opposed to the stock EL34's with about 100 hrs. on them. So, are the JJ electronics tubes going to get better with time?
What brand were the stock EL34s? If they were NOS Mullards (pretty unlikely) then you're new JJs will never come up to that level of quality. If however they were new production (Sovtek, Sino, JJ, EH, etc) then sure, given a few hours the JJs will sound great.
There's no magic to this. All vacuum tubes exhibit a "bathtub" curve of performance over their usable life. There will be an initial change in sound for a week or so, then a long period of unchanging performance, and then one day they'll start to go bad and sound dull and lifeless. Over a very short period they will start to sound awful.
If your amps use adjustable bias you'll note that you need to keep checking it every few hours for the first week or so while the current draw is stabilising. Eventually you won't need to touch it apart from a check once or twice a year. Once you get to that point the valves have stabilised into their final "voice".
