If volume is to be the primary means of contrast, it is best to remain in the realm of flue stops. Reed solos with flue accompaniments step into the realm of contrast by timbre.
A few possible combinations follow, varying from little to moderate contrast in volume:
8' flue solo with softer 8' flue accompaniment
8' and 4' flue solo with softer 8' flue accompaniment
8', 4', 2' solo with softer 8', 4' flue accompaniment