Friend had, prior to his death, the Elekit 300b amp not sure if its the same model. I will say it was a stunning little amp for the money. He used it on his Cornwall (new). He had a local builder assemble the amp he also go the upgrades with it. fantastic little amp if I do say. As for 300b tubes and 2A3's I agree with another poster that the 2A3 is to me a nicer sounding tube but at half the power of the 300b. that said both are some of the best tubes made so its close.
From my experience, after owning Triode-labs 300b and 2A3 amps, the 300b has a better midrange then the 2A3 but the 2A3 has better bass and top end then the 300b. Granted your speakers are efficient enough that the power rating of both is not a concern.
In the end I kept the 2A3 amps and sold the 300b amp. I found the 2A3 was just a more fun tube to listen to, were the 300b was the late night jazz session tube.
I've used both NOS tubes and new production tube, for 300b's the new production Gold lions are really excellent for the money. for 2A3's find a set of RCA black or grey plate NOS those are the ones you'll want, dam nice tube if I do say. I can't afford NOS 300b's anymore lol.