Well, it will be a while before I can get to a full review, since I have the Audio-GD Phoenix and the KingRex Headquarter ahead of it, and since I bought the J Sound Lab myself (meaning it is not a "review unit"), it will take a back seat to the others.
However, after a full weekend of break-in, my initial opinion is as follows: It's a very euphonic sounding amp. This is what people who have never heard tube amps thing tube amps sound like - warm and lush to the point of being syrupy. While not overly bloated or slow, and while having acceptable bass definition and treble extension, this amp should be bought if you want a warm mid-bass and a lush midrange, but if you have headphones that are already a little ripe, it would be a very bad match. This amp sounds great with the DT880 - less so with the DT990, and even less so with the JVC DX1000 where it is just too much.
The amp is 100% hum free and dead-quiet. But it seems to be voiced for a strongly toward sounding "pretty" versus being neutral. It sounds very different from my other "high end" tube amps, which are all pretty neutral. It sounds most like the Musical Paradise MP301, which is also sort of stereotypically tube sounding (and interestingly is also transformer-coupled).
My guess is that the output transformers, which in an amp in this price class can't be very expensive, are what is imparting this sound. I know the 12AT7 itself does not always yield this kind of sound.
So while it's a fun little amp to listen to, it will definitely require very careful headphone matching.
I have ordered some 6CG7's - I will see what if anything (other than more gain) changes when I use those.