If I have a 450W home theater receiver with 5 90W speakers attached to it, can I still hook-up a subwoofer?

Generally the sub-woofer doesn't receive amplification from the receiver, it has its own. It still gets all the preamp stuff like the equalizer. If the connection is separated from the rest of the speakers this is definitely the case, it is called line-level output, and you can use a sub with it if the sub has its own amplifier, which most do. If the sub connection on your receiver is grouped with the rest of the speakers, you probably still can use one, but I would need the model to tell for sure.

Generally the sub-woofer doesn't receive amplification from the receiver, it has its own. It still gets all the preamp stuff like the equalizer. If the connection is separated from the rest of the speakers this is definitely the case, it is called line-level output, and you can use a sub with it if the sub has its own amplifier, which most do. If the sub connection on your receiver is grouped with the rest of the speakers, you probably still can use one, but I would need the model to tell for sure.

Yes the subwoofer is "powered" so it will connect to the subwoofer preamp output. Also just FYI, the speaker power ratings are virtually meaningless. There is no relationship between these ratings and the amplifier power ratings. How many or what speakers you can connect to your amplifier is based on the speaker impedance and the amplifier's current capacity. Just pretend the speaker power numbers do not exist and this will help you avoid confusion. Also, a common misconception is to believe that an amplifier rated for "4 ohms" means that the amplifier has something about it that is 4 ohms. It doesn't, this is just the amplifier current capacity expressed in minimum ohms to make it easier for the consumer to know what speakers are compatible. The actual impedance of the amplifier output is near zero ohms. Anyway, in your case since you are using a powered subwoofer, you are not having any effect on the impedance load on your amplifier. You are connecting it to a preamp level signal that comes before the amplifier inside your receiver. mk

You should be fine as long as you have a real AV receiver from a name-brand company like Yamaha, Denon, Pioneer, Onkyo, Sony, etc. Just run a RCA cable from the subwoofer/LFE output on the receiver to the sub. But - you should do a bit of setup: * Go into the receiver setup menu and make sure all your speakers are defined as SMALL. * In the receiver setup menu - tell it you have a subwoofer * Find the corner of the room with the 2 longest un-broken walls. Measure along the longest wall 1/3 to 2/5 from the corner and put the subwoofer here. You should now run the speaker-calibration step if your receiver has one. If not - research "Speaker Calibration" to learn how to adjust the volume levels so the subwoofer volume is properly matched to the speaker volumes. Hope this helps.

You should be fine as long as you have a real AV receiver from a name-brand company like Yamaha, Denon, Pioneer, Onkyo, Sony, etc. Just run a RCA cable from the subwoofer/LFE output on the receiver to the sub. But - you should do a bit of setup: * Go into the receiver setup menu and make sure all your speakers are defined as SMALL. * In the receiver setup menu - tell it you have a subwoofer * Find the corner of the room with the 2 longest un-broken walls. Measure along the longest wall 1/3 to 2/5 from the corner and put the subwoofer here. You should now run the speaker-calibration step if your receiver has one. If not - research "Speaker Calibration" to learn how to adjust the volume levels so the subwoofer volume is properly matched to the speaker volumes. Hope this helps.