Update : Reply from Sam Pura, CEO, saying that this issue has been reported, but they're not yet sure why it occurs (they are on Mac's, mostly...). As a work-around, he suggested that setting Reaper's sample rate to 48000 should work. I did this, and, sure enough, it now rings out, clear as a bell..! What effect this may have on existing projects, with other sample rates, I don't know, but this snare now sounds, and sounds fine. Very detailed, with a wealth of presets for me to wade through; I'm not sure yet how it will fit in with my compositions, but it certainly has a quality sound.
Thanks, then, for the 'heads up'; hoping there'll be an evolution rendering messing about redundant; in the meantime, I've a lot of snare stuff to listen to, now. Cheers.