There is no way that 5/7 amps are cheaper than one, such as an Outlaw Model 750/770 for example. If the powered speakers are cheaper than the equivalent amp/speaker combo then corners were cut.
If you really like this idea you might check out ACI, another internet only outlaw.
http://www.audioc.com/
They has a powered speaker line.
The only major sonic advantages come with bi-amp or tri-amp using active crossovers and multiple amps. Linkwitz has some good info.
http://www.linkwitzlab.com/speakers.htm