miden,
The Z-5500 system is 500-watts RMS, 1,010-watts peak power, and the output inpedience range is 4 to 8-ohms, which is pretty much standard for this type of system. I agree, however, that you could potentially overload the system by placeing too many speakers in parallel. If you wish to use more than a pair of 8-ohm speakers, you will have to go to a series/parallel hookup that involves four speakers. At this point the impedience will be correct, but the ability to drive all four speakers is somewhat diminshed by the overall load.
For me, the Z-5500 system, with its 5.1 standard hookup works just fine for most jobs. The only thing I'll likely change in the future is the connections to the speakers, plus go to a 6-inch, or possibly 8-inch, lightweight speaker to replace the center speaker. I'll try hooking up one of my 6-inchers in the next few weeks to see if there is much of a difference in mid range sound quality and mid rage volume.
Cheers,
Gary
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Travlin' Easy