I would like to know if it is possible to display the preset number permanently instead of bpm.
No there is no such option available for the display. The tempo (BPM) or the delay time (MS) is showed by the display. However, you can see the preset number for about 1 second by pressing on the "preset button". To change display mode, press and hold the "subdiv" button. The display will change between tempo or delay time.
For further information, please see the user manual located here