5.1 under 4k?

Since you are used to Polk, you'd probably like GoldenEar, which is one of Polk's co-founder's current company. At your budget, you could get three SuperSat 60s for L-C-R ($2400), a pair of SuperSat 50s for the surrounds ($1K) and their Forcefield 5 subwoofer ($999). This all comes in around $4400. I've heard their sats with their sub (as well as the Triton 2 towers and Aon 3 monitors), and their products have a *great* folded ribbon tweeter and their satellites with sub sound really good for both music and theater.

I have had an Onkyo A-9555 integrated amp for five years. For the money it was a good amp, but no way can it approach a Marantz PM8004.