Bookshelf Speakers with Rel 212se Subwoofer

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.