Received a pair of Ken-Rad VT-231 today. These are often costly so I have been waiting for a good deal to pop up on eBay. Even though these look rather different, they are the same tube and sound identical. I suspect many bidders were put-off by their mismatched appearance, which was to my benefit. :)
The black glass seems a tad bit taller and was manufactured in June, 1944, and the shorter, clear glass, January, 1945. From what I have read, GE took over the company sometime in 1945, and the later, GE/Ken-Rad tubes don't seem to get much love. You can tell the original Ken-Rad from the later GE/Ken-Rad by the orientation of the plates. The original Ken-Rad has "staggered" plates. That is, while the plates are parallel, they are mounted between the micas at an angle. You can see this quite easily from the top. The later GE/Ken-Rads also have parallel plates, but they are mounted parallel to the edges of the micas.
This tube is considered to be the "Bass Champion" of the 6SN7s, and I can certainly attest to that. The mids are neutral with a bit of sweetness, and the highs are quite nice and extended, but not as smooth and airy as the Sylvanias. All in all, a very nice tube.