Receiver with assignable amp to power subwoofer

I have an in-ceiling 5.1 speaker system with a passive sub that requires an amplifier. Can anyone recommend a 7.1 receiver that has an assignable amp with a low pass filter that can be assigned to the sub? I would like to keep the cost around $500 new or used. Thank you in advance.
I don't think anyone has an A/V receiver that has enough current to drive typical 4 ohm subs. In the $500 price range, a lot of them that are rated at 100 watts per channel, will only drive one or two speakers at 8 ohms for the 100 watts. Some even drop down to under 10 watts a channels if all 5 or 7 are driven at once into 8 ohms. Maybe that's another reason for the point one sub output. I think the best option is a powered sub, or a separate amp that can drive your sub. This is an example of how receivers ratings are getting. Nothing like the hi-end we're used to. Link []
Just get a pro mono amp for it, plenty of power and cool runners can be found super cheap.
Not a lot of passive subs out there in this day and age.....just do as Chadnliz says and buy an amp.