OK so lets start at the top. If we start with the Pre/amp combo
one choice would be Arcam AVP700 Controller and P1000 Multichannel Amp
Together that would total about $4500 retail leaving $500 for a sub.
Parasound 5125 $1800
Model 7100 Surround Controller, $3,000
That might be cutting it close. But remember these are list prices. You should be able to get a better deal at the store.
Or I know that Outlaw audio should be fine as well
You could get
That would leave over $2000 for a sub.
Here are some linkshttp://www.outlawaudio.com/http://www.parasound.com/http://www.arcam.co.uk/
Now for subs in the $500 dollar range you could look at Outlaw, HSU, SVS.
If you want to go the outlaw electronics way, there are a ton of options. If that is what you want, then we can talk about that as well.