David: It's definitely NOT rated for 4 ohm speakers. Very few receivers are. (This is due to UL/CSA heat safety disipation concerns.)
The question is, can you still use it for 4 ohm speakers?
Yes you can, but you need to think about the setup a bit.
If you are driving speakers that have sensitivity ratings below 90 db/watt/meter or so, then they may try to pull more from the 509 than it can deliver. It would likely overheat and die... On the other hand, if you run higher sensitivity speakers (or use lower spec'd ones and don't plan on playing your setup very loud (which makes little sense for a home theater)) you might get along fine, as long as the unit has good ventilation.
I have a month old 509 that I'm running with 4 ohm, 87 db/watt/meter Thiels. So far so good. But I don't shake the walls with this system and a month is not any kind of long term test.
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search for my name you'll find the info you want.
kenwood told me they cant handle 4ohm but a person from THX told me the 510 can run any THX select speaker (even 4ohm)
It'll run it just not 100%
Get it you'll be hapy
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Evan Adams [email protected]
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Get a high end set of speakers that carry a high sensitivity
and Ohm load won't mean much.
For example I run AR9's which spec out at 94Db Sensitivity
and they are labled as 8ohm load. Pushing a true 100watts
full bandwidth from an Onkyo DS787 I can not go beyond 20Db
on the volume scale (which maxes out at 70Db) If I go higher
than 20Db my ears start to ache and I slip in and out of
conciousness =)