green mountains

I don't know how else to say this ,but the Minis absolutely blew the Europa away in every regard. You may want to look at Aether Audio's Spirit 1. (recommended an amp with 200 watts with the Mini)
I sold these after 3 fantastic years. I could have lived forever with the Minis, but I wanted to try a new slightly smaller stand mount.
I am waiting to receive the Vapor Audio Breeze which I plan to pick up at Akfest in April. The Breeze lists at only $1250 in standard finish. Ryan custom builds each speaker, so I chose a custom redwood burl,curved cabinet,with matching stands. Vapor uses the outstanding Raal tweeter and a very well regarded woofer in the Wavcor.
There is a very nice pair of Volents vl2 used here, for an amazing price. I have always coveted this speaker. Read the 6moons review. I would look real hard at this monitor. They are efficient, gorgeous , amazing bass, ribbon tweeter etc...
You may want to widen your search as there are many great monitors out there, and it may be fun to try different manufacturer.Please note, I'm not saying the Rio and Eos are not fantastic speakers. In you price range I would also have a look at Fritz carbon 7 or 5, and Selah Audio.
Good luck in your search.

First of all what kind of music do you listen to?

And can you describe in more detail the experience you had with the Europas and the Eos/Rio? What were their strengths/ weaknesses? How fussy were they with positioning? What was the tonality like?

I was initially drawn to the Green mountains because of the advertised claims of time coherency which other speakers do not have. What can you say about this? Are there any qualities which are audible with GMA that are lacking in others?

I will let others with more experience with Green Mountains give their impressions. This was my first real audiophile speaker.
The way I understand it, first order crossover speakers are more crucial to place correctly, and have a smaller sweetspot. I may be wrong on that.
I think if you want better feedback you should list what kind of music you listen to, what kind of spl levels you listen to, room size, amp to be used, and if you plan on running a subwoofer.
If you love the sound of your speakers, maybe moving up the line is great move for you.
I just think there are many great options for monitors for under $2500 especially when you are considering buying used.

I have not heard the particular speakers you asked about, but I have heard and owned quite a few of Roy's designs.

Once you hear time and phase coherency done right, it's hard to go back to anything else. I currently have the Continuum 3 HDs and I believe them to be fantastic speakers. I don't think there is anything that I could afford that can beat them. Something will, for sure, but it's probably out of my reach.

As mentioned earlier, give Roy a call. He is always ready to talk about his speakers.

Shakey

I haven't heard the Rios or Europas, but I owned a pair of Pico Executive HDs last year. They throw a very nice, detailed 3-D soundstage with excellent pinpoint imaging. Tonal richness and timbral accuracy is fantastic, and the sound is very smooth, pleasant and engaging.

The phase-coherent design I also liked, and I think it helped enhance the front-to-back dimensionality of the presentation. I didn't think they were any more susceptible to a small sweet spot than lots of other speakers - perfectly easy to enjoy them while sitting in a chair off to the side.

That said, the imaging and soundstage are liable to be on the smaller side. The GMA designs generally don't provide a lot of bass and aren't the last word when it comes to dynamics. They're certainly not below-average, either, but I think they lean more toward sounding smooth and beautiful, as opposed to "live" and "exciting." Mind you, I imagine that could change with a more powerful amp than what I was using.