Can I use a 2ohms speaker with a F90BT or do I need to stick with a 4ohms speaker?

4 ohm for front and rear only. 2 ohm permissible on the sub channel.

if you try to run a two ohm speaker off of every channel, it will damage the unit. you need to go with 4 ohm or higher to be safe, if you want to drive 2 ohm speakers you will need to get a high current amp (almost all four channel amps should have no problem driving a 2 ohm speaker)

just for shits and giggles, where did you come across a 2 ohm speaker(non subwoofer? is it from a factory bose system or other premium sound system?