You may need to experiment to see what you like but like most things that are audio related you'll get a lot of opinions. Speaking of more opinions, I find that the center channel detracts from the listening experience instead of helping it when listening to music. A nice pair of front speakers with good imaging will negate the need for a center channel. You may be better of investing in room treatments and experimenting with speaker placement in your room to improve the audio quality than to add the center channel. That said, I know nothing about the speakers you have, so it's possible they may benefit from a center channel more than other front speakers.
It sounds like you would want a DAC without amplifiers if you plan to use an Emotiva amp. This depends on how many use cases you want to support in terms of the source input. You mentioned a computer, TV, etc, but each of those may offer different output sources. For example on the PC, you could use HDMI, SPDIF, or even the line out as a source. If you're using a TV, you'd likely be using SPDIF out or possible HDMI ARC if your TV supports either of those. My point being, if you need a flexible amount of source types, you may need to consider a receiver or a DAC that has multiple input types but also has preamp outputs if you want to use a specific dedicated amp like the X series sold by Emotiva. In my specific setup, I use a Marantz AV7005 Preamp/Processor to manage the various input sources. This device then outputs the preamps to the amplifiers as it has no amplifiers built into it. This does the DAC of the various source formats and does bass management for my subwoofer in a 2.1 setup.
The Yamaha RX-A670 you listed has no pre-amp output (except for the sub) so you wouldn't be able to utilize any of the Emotive amps or any amp for that matter (at least not cleanly). You would need to rely entirely on their built-in amplification for your speakers.