Forever speakers around $5000

new: Salk Song3 BeAT 

Used: Proac D30R/D48R

There are lots of smallish floor-standers available that are very good new and used in this price range. Not sure if the will be forever quality by my standards but we all have different tastes.

I like monitors with with one or two subwoofers. I feel you get a better quality speaker and I tend to like the soundstage projected by a monitor better than a floor-stander. In my experience, it is uncommon that the optimal placement for bass reproduction in a room is the same as mid and treble reproduction. You usually have to treat a room pretty significantly to get that to happen. By de-coupling them, you can get better performance with less room treatment.  

This is my opinion and I suppose is worth what you paid for it.  To that end, the rest of this post is focused on monitors.  

Regarding R&D, larger companies tend to spend money on driver development. B&W, Focal and Paradigm all have unique drivers. They all have mundane (MDF) cabinet materials that are shaped in interesting ways. Some speakers in the space can be kind-bogglingly good. Raidho for instance.

A few smaller companies either use off the shelf drivers or drivers they have modified from a manufacturer like (SEAS, Scanspeak, Eton, Accuton) and invest heavily in R&D on cabinets. Wilson, Magico, Kaiser, Verdant (my company), Marten are examples.

A smaller number have proprietary drivers and extremely innovative cabinets. Vivid and Wilson Benesch are the two I can think of.

In your price range, you should be able to find a pair of Vivid Ovals. These have a bold design but are awesome speakers. Fiberglass cabinets and proprietary drivers, Vivid is one of the most innovative companies in the market. You will need a sub for them. There is also going to be a Kaya S15 coming soon. Not sure about the price, you will need a sub but they will be great speakers.

You ought to be able to find a pair of Raidho X-1s used for less than $5K and you might find C1.1s for about $5K. Raidho’s tweeter is what makes these special and Raidho’s soundstage is addictive. But, you must have 9’ between the speakers and 3’ from the back wall to get the most from them. You will also need a subwoofer with them.

I make a speaker called the Verdant Nightshade 1. It has a fiberglass cabinet with a DuPont Nomex honeycomb core. I use Eton Drivers that were selected based on how they react with the fiberglass cabinet. They are $5K, deliver a huge soundstage and work best with solid state amplification. But, mediocre bass response and will need a subwoofer to get the most from them.

There are other monitors that that will deliver much better bass response and are very good sounding. You may even be able to live with them without a sub. Check out the Focal Sopra 1, B&W 805 D3, Joseph Audio Pulsar, Magico A1: All are above your price range but used will be in your price range. I have not heard the Magico A1.  It is new and to me the most exciting speaker on this list as Magico has a very innovative cabinet design and a great sound profile though I don’t know how that will come to life in a monitor.  

Good luck.

Your current and planned upgrade amp will both drive Vandersteen 3A signature, massive and tight low end - no fatigue mids and highs. Or as Bigkidz recommendation the 5a

the Stratos should remain on your list, an excellent power amp and superb SS value, there are of course others..

There are some great speakers mentioned here. Finding the right speaker, IMHO, is the key and the most difficult of all building a great system. I just went through this process.

Here is my recommendation. Brand new pair of Canton Reference 3 DC Speakers. Once Stereophile Class A Speaker, if that means anything to you, reviewed when they were $16K. 

On sale at Accessories4Less at $4k shipped and no taxes. You need to take in account of shipping as well unless shipping is included. Truly a Reference Caliber Speaker and my last speaker.

Good luck on the journey. The review and the purchase link is below. There is no difference between the Reference 3 DC and 3.2 DC but the tweeter and the 3DC has the better tweeter for a thousand dollars less. 

https://www.stereophile.com/floorloudspeakers/canton_reference_32_dc_loudspeaker/#dQY0lBjuUfkR9XeJ.9...

https://www.accessories4less.com/make-a-store/item/cantventoref3dcblka/canton-vento-reference-3-dc-8...

For good sound and great styling, I'd take a look at Salk speakers.