I consider myself a bit of an expert on the KT100, having owned three types over the past ten years. The KT100 is billed as a drop-in replacement for KT88's in most amps; I never had any problems substituting them in my amps but YMMV.
There are three different KT100s that I'm aware of: the first, and easily the worst, was a generic Chinese version (not sure which factory it came from) circa 1990; it was a KT88 short bottle manufactured with thicker glass to cut down on breakage during overseas shipping. They were OEM for Houston and (I think) Top Gun Chinese amps. They sounded pretty bad, rather like a Penta KT88, but more pinched-sounding and with subjectively less output. The somewhat poor sound was offset by a little better reliability than other Chinese KT88s of the day which had a fairly well-deserved reputation as firecrackers (although they are MUCH more reliable today).
The second KT100 was imported by Tubes by Design (aka VAC) circa 1992. This variety came from the Lizhou factory in China and was built to the very highest quality control standards (supervised by some of the British GEC engineers). The bottles on these monsters were HUGE... they dwarfed tall-bottle KT88s and made short bottles look like toys. They were quite expensive even by today's standards: somewhere in the neighborhood of $500-$600 quad if I remember correctly. But well worth it; the sound was better than any KT88 I've ever heard (and I've heard quite a few of them). BIG-sounding, nuanced tubes with excellent extension, focus and a lush, creamy midrange to die for. All but impossible to find nowadays; I've only seen one pair for sale anywhere and that was at least five years ago (and yes, I bought 'em).
The last KT100 I'm aware of is the Valve Art, circa 1998, which was released as the "KT88-T" and then renamed KT100 to avoid confusion with VA's standard KT88. If I remember correctly both of these came from the Shuguang factory. The Valve Art has it's fans; in my amps both the VA KT88 and KT100 sounded rather soggy and unfocused, but again YMMV. (I would never tell anyone which tube they would like and recommend you take my subjective listening impressions with several grains of salt).
I own (or have owned) several brands of KT88/KT90/KT100s over the years including GEC, Svet, Sovtek and EH, JJ, Ei, Valve Art, Golden Dragon, some generic and/or unlabeled Chinese (Groove Tube, Penta, etc); the early GEC (not later versions) are fantastic but my heart still belongs to the Golden Dragon KT100 which, alas, is apparently gone forever.
I'm sure there are other KT88/KT100 enthusiasts who could add to, modify or even correct this information; it is merely the result of my own experience with this particular tube over the years and others may have different experiences or impressions which I hope they will share.
