The RE9000 is simple to set up. As noted earlier, if you want the RE9000 to use its four-stream radio as backhaul, set your router's 5 GHz channel to 36 - 48 before you start the RE9000 setup.
You can try flying blind and using the RE9000's WPS pushbutton to link to your WPS-enabled router and watching to see if the RE9000's front panel LED eventually turns solid green. This method will set the RE9000 to clone your router's SSID, with _Ext added, and WPA2 password on all radios.
If you'd rather see what's going on during setup or make some changes, you can connect to the RE9000 from an Ethernet-connected computer or you connect via Wi-Fi to the default Linksys Extender Setup SSID. Either way, set your device's IP address to the 192.168.1.X subnet first. Once connected, launch your browser and enter http://extender.linksys.com (or http://192.168.1.1 if that doesn't work) to launch the setup wizard.
Make sure you have not connected the RE9000 to your LAN via Ethernet, or you'll you'll get an online version of the extender.linksys.com site, which hasn't been updated to handle the RE9000 yet.
Unhelpful nline extender.linksys.com page
If you're properly connected to the RE9000, you'll see the setup wizard, the first two pages of which are below.
Linksys RE9000 setup wizard start
Do yourself a favor and assign a static IP address to the RE9000 before you finish setup. It will make things a lot easier should you need to make settings changes later. Make sure you choose an IP address outside your main router's DHCP server range to avoid an IP address collision.
The RE9000's feature set has what you need to configure it for either access point or wireless extender duties. It provides two tools that will be more helpful than trying to use the front panel LED to determine optimum placement.
The SpotFinder provides a simple placement tool for folks who don't want to mess with interpreting signal strength and link rate to find the right place to park their extender.
Linksys RE9000 SpotFinder
If you are someone who wants to know what's happening in the backhaul link, the Upstream Network Statistics page is for you. It contains channel, bandwidth, link rate, signal strength and signal quality information. Very helpful.
You can see how the fronthaul/client connect radios are set from the Extended Wireless Statistics page.
Fronthaul/client connect info
Most settings are found on the Basic Settings page. The top half of the page holds controls for LAN address and backhaul radio...
Basic settings - LAN and backhaul radio
...and the lower half contains fronthaul/client connect radio, host name and clock settings and a Reboot button. When the Band Steering box is checked, the same SSID and WPA/WPA2 password is set on both radios and the channel and channel width settings are greyed-out.
Basic settings - fronthaul radio and other
Unchecking the Band Steering box, exposes controls for each radio.
Fronthaul/client connect radio settings
I checked for a hidden advanced wireless settings page and found one. But you'd best leave most of these settings alone. The only ones I'd mess with are the Turn on/off wireless, but I don't know why you would.
Hidden advanced wireless settings
Two other RE9000 features of note include access control and 802.11r support. MAC address based access control can be set to either allow or deny devices. Enabling this feature disables WPS, which is enabled by default. 802.11r (fast transition) support is disabled by default, but can be enabled on each fronthaul radio. Unless all your devices support 11r, however, leave this disabled or devices that do not support 11r won't be able to connect.
Unlike NETGEAR, Linksys doesn't force you to register their products. And if you do register, you don't have to use your email address as your username.