Just gave the cat a blast of pink noise 25k-48k and she didn't bat an eyelid. Either it's not there or she can't hear it - and from what you say I'd tend to go with the former.
Well, the text in the original BBC Research Report says "Also, its output falls off somewhat above 15kHz, but for the purposes of broadcast monitoring, this is of no significance"
(bottom of page 2). But if you look at a typical response of a real unit, they hold up
fairly well to around 20kHz - it's after this that I believe that things deteriorate somewhat.
And for anybody else wondering, you have to bear in mind that we are talking about a pretty serious monitoring system here - in absolute terms an LS5/8 is no slouch at all. If you want a significant improvement in overall performance, you have to pay thousands, or do a lot of your own research - and still pay huge sums for the drive units. And when I said 'hold up pretty well', I meant within a couple of dB of the response at 1kHz, and pretty much flat all the way there. And despite being no youngster, this is a proper bi-amped system with an electronic crossover, and no, that's
not how they extended the frequency response - the R&D department really was being a bit picky about this aspect of its performance.