The JBL/Reaction system you outlined looks really good. Between the Marantz and Denon x2100, you can't go wrong with either, they are more alike than not. They are both from the same company, D+M Holdings, they both support full 4k resolutions, they both have very good room equalization. You might as well flip a coin between them. If it were me I would go with the Marantz and only because it looks a bit nicer.
With respect to the subwoofer, the Gamma 15 is a very good one, but I would go with a ported model instead of the sealed model. I originally linked to the Reaction Echo 15 but labeled that link the Gamma 15, my mistake. Sealed subs like the Gamma 15 sacrifice deep bass for a smaller cabinet size, but if you can handle the larger cabinets of ported subwoofers, you will be rewarded with a lot more deep bass output. The subs I would go for are the
Echo 15 or the
Hsu VTF3 mk5.
If you wanted towers instead of bookshelf speakers, I would be looking at the
EMP Tek R55Ti tower speakers, with their
corresponding center speaker, and use the
R5Bi bookshelf speakers for surrounds. Those towers are very affordable, with good sound quality and they look nice as well. They are highly regarded by audio buffs as an inexpensive tower speaker.
For the bookshelf speaker surrounds, I would use the
Videosecu side-clamping bookshelf speaker mount, which are by far the best bookshelf speaker mounts for the price. Get your HDMI cable, speaker cable, and subwoofer cable from a company like
blue jeans or
monoprice; do NOT buy cables at a store like BestBuy, because the markup on those items at electronics stores is usually really bad.
