Yes, in the 50s Jay and Kai were very popular by today's jazz standards - not the popularity of Elvis, of course, nor even the popularity of Brubeck or the MJQ for that matter - but they were big enough for a 17-year-old friend of mine to take up trombone in an attempt to emulate them. Anything that can get through to someone of 17 must be called popular, don't you think? Don't forget their arrangements were catchy, almost singable, and their varied tonal effects - with mutes, etc - helped make an attractive product. I'm well aware of this as I frequently go to hear a trombone duo who do a Jay and Kai show with transcribed arrangements and they're very easy on the ear and get a big reception from an audience, many of whom must have been teenagers in the 50s.