On the Herford 67/XL I use
For the Rec: Choir: Open Diapason 8, Gemshorn 4 Spitz Flute 2 Mixture III
Pedal: Bourdon 16, to choir
For the Pos: Solo: Tromba 8 (box half open)
I've found this to be quite satisfactory if perhaps a little "English".
I've also tried the Great and Swell Trumpets for the Positive ("melody") but prefer the smoothness of the Solo Tromba. The
I have recordings from Coventry Cathedral (Harrison & Harrison) and St Pauls (Willis/Mander) which both seem to use (different) smooth reeds for the Positive part. The St Paul's recording is quite slow.
Iain