Best Smaller Bookshelf Speakers For Use Exclusively With Ambient Music?

Ascend Acoustics Sierra2.

SEAS woofer with RAAL ribbon tweeter in a bamboo cabinet. Around 7-8 hundred used. A lot of speaker for the money. There's a pair for sale now on USAM.

while not much thump, I find ambient music often has lots of deep bass...I'm currently using Audience 1+1 and REL T/Zero lll in room same size as yours...I found used for under your price limit...

if a rear ported speaker is okay look at a used Ref 3A de Capo.  detailed, rich sound

Totum Mite (already mentioned) is worth looking into as well as Totum Sky.

Sonus Faber Lumina I or II might work depending on the amp.

I like the small Silverline and Reynaud speakers, but did not see any for sale on the used market.

There is one pair of Reynaud Twins MKII listed as "pickup only", but the ad has been up for 6 months (I'm 99% certain that they are my old pair -  per the pics/slight damage to one back corner).

DeKay

Enya was never an ambient. She's all the way pop singer.

I'd look for beoplay active speakers. You can get one or two and synch between them to stream music to both of them.