Set it up this morning A/B with one of my pa units, which is an RCF EVOX 8. Much the same gubbins inside, I imagine, aside from the cross over. Sounded just the same. Low-ish volume on 'line' but, set on 'mic', and using the DI pedal, I got plenty of volume. No idea what I was doing wrong yesterday but clearly I am a dimwit.
I wonder if I can send me back to Thomann for a refund?
Anyway, all's well that ends well and thanks for the help!