potentially, yes - but why would you do that? Just plug the headphones into the source.

The main advantage is that this isn't just streaming from the phone, the phone just tells the chromecast what to play, and the chromecast itself streams directly from the internet rather than your phone. That means the soundbase wouldn't be reliant on your phone once the music is started, you can be doing other things on the phone while the music still plays through the chromecast. Basically the phone/computer becomes a remote control for the chromcast, rather than being the actual device playing the music.

Far far better quality than bluetooth. plus you can use phone as normal whilst streaming

No, that's not how they work. You stream the playlist / grant access to accounts and the device pulls music from the internet itself *via wifi*.
To have it work in your car it'd have to have access to a local wifi or you'd have to tether to your phone. Tethering data allowances are a bit fiddly.
If you're going to do that, you may as well get a bluetooth to jack adaptor or just plug your phone straight in. Unless you have an iPhone 7 (lol)

​It will work in a car via the aux input but it needs a power source, a phone power bank would do that ok. The Chromecast Audio works over WiFi rather than Bluetooth, should work fine from any smartphone.

Something like this?
https://www.amazon.co.uk/EDIFIER-EDFR980T-Home-Audio-Speakers/dp/B00GBN4TNO/

https://www.amazon.co.uk/LEPY-2020A-Amplifier/dp/B00LEHBDCW/ref=sr_1_4?ie=UTF8&qid=1477952975&sr=8-4

PLUS

https://www.amazon.co.uk/Roth-Audio-Bookshelf-Design-Speakers/dp/B00G3C8BTQ/ref=cm_cr_arp_d_product_sims?ie=UTF8

The volume and bass will be underwhelming, but if you're getting a high quality output I don't think you'll beat it for £100 new.

​Bose Companion 2 Series III, £75 on Amazon. Small but mighty!

This is what I plan on doing. Obviously need to get power into the ceiling though. Was just going to have this and a mini amp to live in the ceiling next to one of the speakers..

​Very interesting, thanks.

Also, the price is now £24.99. Think I may just wait for a price drop and for the BT router issue to be resolved now.

Buy this http://www.richersounds.com/package/system-savers/mini-hi-fi-system-deals/pah012735 and a audio chromecast and you will be set, can be found cheaper if you search but be careful as a lot you see at a cheaper price dont come with speakers ..

http://www.whathifi.com/denon/d-m40dab/review

This device needs it's own power supply, so probably not much use for your portable speaker. That said, the Chromecast Audio (CCA) takes a standard USB-MicroUSB cable so, theoretically, you could have it hooked up to a portable phone charger.
You need to be in range of your WiFi though.

These devices are fantastic though and are easy to set up and breathe new life into an old Hi-Fi you might have. Think of them as like a Sonos but without the speakers. Very similar functionality. In fact, more so as you can cast your screen to the CCA and play the sound from YouTube videos.

I bought one of these when they first came out and loved it. I got a deal on a Sonos Play:3 which is in my dining room, but my CCA is still hooked up to my home cinema in my livingroom. Basically anything with an Aux input, stereo input (the red & white co-ax plugs) or Optical input (aka TOSLINK) the CCA will hook up to.

If you are using the analog output make sure you enable "Full Dynamic Range", sounds much better. https://support.google.com/chromecast/answer/6290498?hl=en-GB

Funny that a Chromecast Audio just does Audio.

I have 3, each connect to a separate unfii wireless AP and have seen this type of behaviour. Resolved it by enabling IGMP snooping on the network switch (that each AP connected to the LAN via).

Yes