In other words, start with 2.4 ghz first. Use either Inssider or some other similar program either on a laptop or on a phone (Android, Windows, Apple, it doesn't matter). I have a Samsung Galaxy S5, and I use a combination of apps, including Netgear's Wifi Analytics, WiFiOverview 360, Wifi Analyzer and Network Signal Info from KaBits to figure out what channels my neighbors are using, so that I can set my control channel to the one that is least affected by other nearby 2.4 ghz AP's. Remember, use only channels 1, 6 or 11 to avoid overlapping with those who are using the channels properly. Find the channel with the greatest strength and take various measurements from as many locations as you need to in order to optimize the placement of your router.
Contrary to someone's suggestion above, I would set the 2.4 channel to use 20/40 MHz width since the channel width will automatically drop down to 20mhz if there is any interference from other nearby routers. Or you can just leave it on "Auto".
Same thing with the 5.0 ghz band-- find the least congested channel you can find and find the best location based on the signal strength you see using Inssider or the other similar apps. If you have no neighbors using any of the 5.0 ghz channels, I would feel free to use 80mhz as the channel width. If the area is crowded with other 5.0ghz users, then be smart about it, and set the width accordingly. I personally have had great signal strength and range using channel 161 and 80mhz, but your situation could be different from mine (there's only one neighbor a good distance away, and he's using only one of the lower channels so we're not even close to each other).
You do need to position your router in the most central area of your home for the best coverage. Others are right, you'll be better off if the router is upstairs rather than in a basement. Also, you might want to adjust the transmit power (look in the GUI settings for "LAN >"Professional" and set the transmit power to either 80 or 100mw. If you're using the Merlin fork as you report, that should be fine, but if you want to experiment, you might try to roll back to an earlier Merlin FW, like 374.38_2, which has worked great for me and which I continue to use.
A lot will be determined by the construction of your home and the walls in it, as well as what else you're using in the way of electronic devices. And believe it or not, your wireless range will be affected by people in your home as well (think of them as bags of water, through which the 2.4 and 5.0 radio waves don't easily travel). Likewise, if you have an older portable phone system, it can be interfering with the 2.4 band, as can a microwave, a baby monitor or other devices that transmit within that band. There's not much that can screw up 5.0 ghz at the moment (other than certain civil defense and military uses that can preempt your use of some channels on the 5.0 band), but you should also realize that because 5.0 is a higher frequency, it won't travel as far as a lower frequency like 2.4 ghz, and your "range" of each will not be comparable at the outer edges. Think of the two bands like you would of a woofer and tweeter...you can hear the bass from a woofer a lot farther away than you can hear very high notes from a tweeter, because the lower notes (which are at a lower frequency) travel farther than the higher notes, which are at a higher frequency...same principle. If you have a lot of brick, masonry or metal in your home's construction, that can also change the way the signals behave in your environment and can limit or negatively impact your wireless range.
The only way to really resolve what you perceive to be a wireless "range" issue is to experiment and try to position your router and clients in optimal settings.
Also, if you're not getting great coverage on a second floor (or a floor that is different than the one your router is located on) you might want to consider getting a second router and setting it up as an AP (to extend your wireless signal to hard to reach places), or if you can't run cable between the router and an AP, you can do what I have done: Purchase a second AC66U or R, and set it up as a "repeater". There can be some tradeoffs if you can't position our second unit within excellent wireless range of the router, but if you can, it's a great solution and not too expensive.
Be sure that the clients you are using can actually connect to the bands you are trying to use. Obviously, a client with an 802.11n dual channel adapter is able to use the 5ghz band, but it won't connect at AC speeds.
For starters, you will want to read some of the basic material on the front of the SNB site, in particular the articles that describe how best to configure your AC router and what to expect from it.
Oh, and if you're not using any of the other features of the AC66 (such as the media and dlna servers, etc.) turn them off in the "USB Settings" menus, both in order to preserve your security and to avoid any unexpected interference that might occur.
Lastly, this is often overlooked, but position your antennas on the router so that they form a "W", with the two ends also angled backwards slightly (about 45 deg.). Works for me with my router on the second floor to increase range downstairs. YMMV.