Marantz SR3001 enough?

Yes,it is, don't expect to blow the roof off. Further along the road, your ears will tell you if your present system begins to lack the spark you desire. As for now, enjoy the sights and sounds.

IMO, not really. You're not going to expect anything spectacular, in terms of real sound quality, until you pick up an outboard amp to go with those speakers! This is fact. The modest receiver IS NOT going to offer you any sort of "mind bogglingly good sound quality", or anything. And why not get that, when for a few hundred bucks more, you can pick up supperior amplification on a used 5 channel amp?!
Picking up a used parasound HCA1206/1205(even better), or modest Rotel 995, and similar, and watch your sound qualiy more than double!!! I kid you not!
It's all about sound qualty with those things. You might as well get the best sound you can get within reach.
I always say...
Too many novices think the recieve they read some stellar review on is the "end all be all" for great sound to impress their friends. It's not the case...trust me

I would be ok for your surrounds but not for mains or center IMO. I run 60w to my surrounds and I run 400w to mains and 150 to center, tho the surrounds dont get a ton of power I have never felt a need to change and I get picky really quick and as cheap as things are used I would have swapped it out long ago.