Bookshelf Speakers with Rel 212se Subwoofer

There are many good book shelf speakers. Look for one that matches your amplification and source and cables. You have plenty of bass so if it were me I would want a speaker with a reach midrange. 

@sounds_real_audio : Did you mean "rich midrange"?

Look at Reference 3A de Capos

never tried them with a sub b/c they have enough bass for me but it would defintely extend the range.

What speakers do you have now, and what improvements/sound characteristics are you looking for?

I have a pair of ATC SCM19's and I just love them. Very tight and accurate, with great imaging and soundstage.

The frequency response rolls off below 58hz, but I am using a bass array so it wasn't an issue.

Gentle and deep.Candy Crush

Lots of good monitors out there so I am sure you will get lots of advice. 
 

Don’t forget to take a look at thin tower speakers too. You might find they blend better with your subs. Sometimes when playing high volumes the mid bass lacks when handing off large subs to small bookshelves. The scale just seems off somehow. Maybe something like the Revel 226be if you are not dead set on monitors. 

@yoby , just this weekend I heard a pair of Dynaudio Heritage Special loudspeakers do a surprisingly excellent job of casting an image while sounding totally neutral. They reminded me of the LS3 5A but with more bass. They are within your price range. IMHE only Harbeth makes bookshelf speakers of that quality. Since you are using a subwoofer the Harbeth P3ESR XD is another option and less expensive. Do not let the small size of these speakers fool you. The Dynaudio has a ferro fluid cooled tweeter. With a subwoofer and a proper two way crossover they will play as loud as any large point source speaker.

Wolf von Langa SERENDIPITY.

@grinnell I have a small REL sub with my de Capos, crossover set pretty high. They blend seamlessly. 

Here's my recommendation: Revival Audio Atalante 3 monitors. 

All the best,
Nonoise

A used pair of Joseph Audio Pulsars are well within your budget and are all-around outstanding speakers that’ll let you cross over your sub at a lower frequency than many other monitors.  Fritz speakers also worth a look.  Best of luck.