Probably, but no need. In theory it might have better range because it probably has equal transmission power, but it's broadcasting on a lower frequency (2.4GHz vs 5GHz) that-- like AM vs. FM-- has longer range and deals better with interference. That is, of course, unless you had already implemented the Beam Forming to specifically direct the signal of that AC router. In reality the last LTT Youtube vids I recall seeing saw a better "effective range" from the AC routers. So, even if they didn't reach as far in terms of absolute range, they were transmitting at a higher bandwidth at the edges of said absolute range. Obviously that is your chief concern, here. It could also simply be a matter of interference. The 2.4GHz band is much busier in most locations. Of course, to counter this, there's also the option with the N66U of buying longer antennae with a higher dBi that project a more powerful signal (via higher efficiency of projection of the transmission power). You just unscrew the old antennae and screw on these replacement aftermarket upgrades. These aren't too expensive, IIRC.
But...I was just casting out some Googles because I could have sworn the AC68U was dual band. You shouldn't have to buy the N66U. I think you should be able to calibrate the AC68U to broadcast an "n" signal on the 2.4GHz band instead of an "ac" signal on the 5GHz band. And...yep, looks like my memory served me well, here. check out this page. It has some screenshots of the router's GUI:
http://www.cnet.com/products/asus-rt-ac68u-dual-band-wireless-ac1900-gigabit-router/
So you should definitely be able to set up an "n" network. It looks like you can set up three different networks, so you don't even have to sacrifice that ac network for your ac-capable devices. You just set up two networks: one on the "ac" standard using the 5GHz band, and one for the "n" standard using the 2.4GHz band. Connect the appropriate devices to the appropriate network. Hell, you could even set up a third "Guest" network with no password (I do this, and I'm chincy with the bandwidth, LOL). Here's a link to the User Manual:
http://content.abt.com/documents/53176/E8119_RT_AC68U_Manual.pdf
The answer should be somewhere in "Setup" in there. The GUI's within the router itself looks pretty intuitive. You should be able to figure it out.