There are lots of discussions about optimal effect order, but it almost always comes down to personal preference. Try them both ways and see what floats your boat.

Having said that, time-based effects like echo and reverb are typically close to the end of your effects chain. Overtone and synthesis type effects are usually close to the beginning of your effects chain. The psychoacoustic effect most people would go for with a B9 pedal and a delay (echo/reverb) unit would be an organ playing in a space – cave, hall, church, etc. That implies that you would put the B9 BEFORE the delay. The B9 turns your guitar into an organ. The delay then makes it sound like that organ is playing in a space. That’s how I would run it – B9 before delay.