what differentiates tube integrated amps?

Kind of a generic question, but there seems to be a fair number of chinese built $1500-2500 el34 or kt88 tube amps such as line magnetic, prima luna/mystere, cayin etc. They seem to appear to have the same features +/- and not too much to distinguish from each other. I guess with anything it's different strokes for different folks, but solid state amps seem to have wildly varying specs at similar pricepoints whereas these amps have similar specs at the similar pricepoints.
As far as specs go, output power on these is limited by the choice of output device. If you limited SS amps to a specific output device, class A excepted, you would find that the output power was similar as well. That's why when Sanken came out with transistors with integral thermal sensors and everyone started using them, all of the amps were around 60wpc.

If you mean feature set, that is related to marketing and most amps, tube or solid-state, don't have tone controls, mono switches, phono stages, power meters or even tape monitors any more.
What the tube amps have that distinguish them apart is there sound. The exact circuit design and quality of parts can make tube amps using the same tubes sound quite different from each other. Power supply design is a major contributor as well as quality of output transformers. Actually most solid state amps have similar specs except for output power. Remember you can get tube amps with output power from 2w to 400w
Also some of it is, I think, because there are OEM manufacturers whose products are marketed under more than one name.
Primaluna and Cayin are or were made in the same factory but to different specs. Primluna is a Dutch company.

All the brands you mentioned are a cut above the cheap Chinese stuff. I have a Primiluna and it is a heavy well built great sounding amp. And I wish I could afford a Line MAgnetic.
I am a fan of the Cayin/VAS stuff. Own two interates and two pairs of mono blocks. Great sound, great value. Cayin/Vas build knock offs of the classic Marantz and McIntosh designs using SOTA components. No need to reinvent the wheel approach.
Pay attention to power supplies for sure. And the quality of transformers.