Rory, I *think* the Piegas roll-off higher than 20kHz... (Heard the 10 once & was impressed, BTW.)
Also, I have not experienced any tonal/ phase/? inaccuracy listening to ribbon tweeters -- but that's just me, no ribbon "guru" here!
OTOH, is the Rowland wide-bandwidth? (I don't think so) Why worry about signals over 20-30kHz if the amp doesn't output them -- or am I missing something? Cheers!