A lot of active speakers are classified as studio monitors and have a few inherent differences when compared to passive speakers. Studio monitors are designed to be very accurate and have flat response curves. They will reveal possible issues with a source and generally have smaller sweet spots (because they are meant to be used in near field with a single listening position). Now this is not true of all active speakers, but a good majority fit the bill.
I use powered speakers at my computer and have been very happy with them, while doing research on my purchase I came to the conclusion that there are more of them suited for nearfield then there are passive speakers. A lot of passive speakers really benefit from a wide spacing allowing the imaging to reach its full potential. This isn't always possible if being used at a desk or in a confined area. You can go to guitar center and demo quite a few different ones, but the sound will always be different in your room.
Upgrading your setup with passive speakers is easier however because you can buy a new amp or speakers and not have to replace everything. One thing to factor in to your decision however is volume control. A lot of active speakers have separate volume controls on each speaker, on the back. This necessitates the use of a mixer or audio interface to provide intermediate volume control. Using the digital control on a computer is an option, but you run the risk of bit loss when the computer volume is not at 100%.
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