If your C90 is 4 ohms, yes, you can. If it is 8 ohms then you would use a 4 ohm setting if you are running another 8 ohm speaker at the same time. The other speaker would be plugged into a 4 ohm jack as well. Otherwise, you would run a typical 8 ohm speaker in the typical 8 ohm jack. This is not to say that you can't run it in 4 ohms but it will be darker. A mismatch to a higher ohm load isn't a bad thing for your amp. It is a mismatch to a lower ohm load that is bad for your amp.
