I forget the 'XR' tweeter model but the reason the 'HF1300' was inadiquate is it would only reach around 15KHz (i think, maybe 18kHz), most companies like Spendor with the BC1, B&W with the 'DM2/4' etc would use a combo of 'HF1300' & Coles '4001'... the coles tweeter alone has a spec that will reach around 35kHz...It's much cheaper i'd have thought to just make a soft dome & plonk it in a revised 15..

http://www.troelsgravesen.dk/vintageBC1.htm