Amazon abilities but with digital output... Sonos?

@dtximages  - As @mattmiller indicated you can use a 1/8 audio jack to RCA inputs cable with most Echo devices.  You can also use a Sonos Port for a Alexa controllable source, but at $450, it is a bit pricy.  Both Bluesound, through BluOS, and WiiM units are now Alexa enabled.  You could pick up an older used Bluesound Node or 2i for a couple of hundred bucks on eBay or USAudioMart and they have optical, coax, and RCA outputs.  A WiiM Pro (with the same outputs) is $150 on Amazon.  

Now, this all assumes that the "main" system is being controlled separately and can be set to default to the desired input and/or volume setting.  There are ways to use an Alexa compatible smart outlet to turning on/off an system.  I have done this with systems in my home as well has helped others to do it.  But I would need more info on the equipment of the "main" system and what sound quality is desired, etc.

- Jeff

From the Bluesound web site:

Bluesound now supports voice control. Amazon’s Alexa is the first voice integration for BluOS, allowing you to use your voice to explore music services, select presets, adjust volume levels, switch up songs in different rooms, or even group your Players together and play your music in perfect sync throughout the house. Simply connect an Alexa device to your Bluesound system and command your music like never before.

From WiiM website:

Easily build your multiroom sound system with your existing AirPlay 2 speakers, Amazon Echo, and Google Home devices. Using your legacy audio devices, you can create a more flexible multiroom sound system with multiple WiiM Pros, WiiM Minis, or Linkplay-powered devices. WiiM Pro supports multiroom from various audio source inputs, including WiFi, BT, Aux, and SPDIF input, making it versatile with almost every music listening option.

@dtximages -  As far as I know, and from looking at the Eversolo web site, the Eversolo devices do not integrate with Amazon Alexa.  So with that out, and not wanting to use the 1/8' jack out of the Echo devices, off the top of my head, that leaves only a few options...

1. You could try and find an older Echo Link on eBay and use the optical or coax out
2. The Bluesound skill which you don't like.  That's ok...
3. You didn't specify what Eversolo you have, but if it has an HDMI Input, you could hook up either a Amazon Fire TV Cube or Firestick to the input and at least use it with Amazon Music.  I believe the A8 and the A10 have HDMI input. That would also work if your preamp/receiver/processor has a HDMI input.  I've not tried other streaming services with the Fire Cube/Stick, but it might be worth a try.
4. The WiiM Pro.  It is supposed to integrate directly with the Alexa enviroment.  I just bought one for the garage system and haven't played with that feature yet.  I do know one individual that uses it to turn on/off other AV equipment in his rack with its trigger out connection.
5. Use a Sonos Connect as you first suggested.  I've done it at my house, but it was a bit iffy and it forces you to run two control systems (Alexa and the Sonos App).  (Oh, and make sure you set the Sonos Connect output to "Fixed" and not "Variable" or you will give yourself a few ugly suprises with the volume knob.)

Hope that helps....

- Jeff