Ear level *WHEN YOU ARE STANDING* esp if you are using direct radiating speakers. The object of surround speakers is to reproduce a diffuse sound. If you are using direct radiating speakers at ear level when seated the sound level has to lowered to such an extent that the diffuse sound effects that a surround is supposed to reproduce will be compromised. Same ear level when standing rule applies to dipolar speakers. A good set of dipoles at the correct height are a very worthwhile investment.
Agree totally with Rick regarding speaker cable.