But if you already own an AVR, I'd rather spend $500.- on a stereo amplifier for the fronts and music and leave the smaller effect-channels to the AVR.
Expanding on your excellent approach,
@martijn86 , I'm surprised that more members here don't rely on a high quality two channel system for the fronts and separate three/five channel amplification that drives center and surround speakers - that should be selected to have sonic a signature similar to the front speakers. If I had a larger room, I would consider a basic AV preamp like the
Optoma (Nuforce) AVP-18 ($700) or possibly the Outlaw 975 ($550 for 1080p or $700 with 4K video add-on). Images of both below...
Using an AV preamp and dedicated two-channel amplification for the front speakers with a multi-channel "sub-system" seems like a logical option, especially for those on a budget. One could use a line-level RCA (or XLR) 2-channel source switch between either of two "sources" - RCA output from an AV preamp (which functions as an HDMI "audio extractor" and DAC - or from your music-only high-fidelity music system DAC/preamp. This "selectable" 2-channel source would be connected to the high-quality amp that drives the main speakers. When switched to "AV", the AV preamp would feed your front-channel audio from the video source to the "good" stereo amplifier, and also drive your center and surrounds through the lower-quality multichannel amplifier.
My preference for AV component design is to have it as simple as possible. I don't like rows of extra connectors that I will never use. And I still don't understand the need for component, composite and S-video inputs on the below Outlaw preamp (which also includes an AM/FM tuner). However, I really do like the clean design of the Optoma/Nuforce unit, which has just the basic modern necessities - four HDMI inputs and one output, and RCA outputs for all of the audio channels. The Optoma/Nuforce design is at least 6 years old, but it seems to have a unique blend of features that really appeal to me.
If I were looking for a budget AV system, I would seriously consider a combination of the AVP-18 and the Emotiva 5-channel amp that is the subject of this thread - or any decent budget 5-channel amp, since 3-channel amps are rare. One could just not use the 2 extra channels - or add a pair of side speakers that match your surrounds.
(It is difficult to find low-powered 3-channel amplifiers suitable for only surrounds and a center speaker, but 5-channel amps are quite common. The 3-channel amps I found while doing research for this post seem to be higher-powered for front and center speakers, or the front speakers and a subwoofer - but there are some excellent higher-powered 3-channel amps that show up regularly on the used market.)
For me, when I watch movies and other videos (excluding music videos), the music simply adds to the breadth and depth of the experience, and I focus on what is happening on the screen, and not the background music. Therefore, for me, sound quality for the surrounds would not be as important as for my main speakers. However, it would seem logical to have a center speaker with an excellent midrange to enhance the clarity and intelligibility of the spoken word.
I would like to see Amir test this unit, but I doubt that any member in the U.S. has one, and is willing to send it to Seattle for testing, even though it only weighs 4.6 pounds.
(@Kal Rubinson did a "Music in the Round" review of this AV preamp back in 2013, and found it to be a good budget AV component, but with some flaws.)
But of course, not all AV Preamps are basic and simple - here's a $2,200, 11.2 channel Marantz 7703 :
Based on this measurement, seems like there is no performance spec for each channel. I should say that I find channel differential often but this one is rather extreme. 
