Yeah, it's probably the amp. That's what we get with receivers, weaker amps. Also, you'll be able to get more volume with more speakers for surround. Do you have a sub? Pulling the bass out of your receiver's amps will give you a bit more headroom, too.

Get a meter, they're under $40. Well worth it. Then you can set your speaker levels correctly and see what exactly you are doing in terms of volume, always pays to protect your ears a bit.

My kids wanted it loud this weekend, so I played my favorite THX trailer at 0. Oops, too loud. Scared the crap out of them! Didn't actually measure it, though.