Speakers for Leben CS300XS

Proac D30R or Devore 0/96 are both outstanding.  Smaller versions of those two brands, the D20R and 0/93 should also sound great but I have not heard them.  
You have one of my favorite amps that I don’t own.  If I had to start all over again, the combo of your amp and the speakers I named would be at the top of my list.
None of these speakers are inexpensive.  Hopefully, they are something you can swing financially.  You didn’t list a budget.
AZ Hi Fi is my local dealer.  They may have other suggestions as well.
No affiliation with them other than a satisfied customer.

I don't have a Leben but I'm running a 17 watt EL34 amp into a pair of Audio Note AN-J's.  At 93db sensitivity and a benign impendance they would be a good match for your Leben and sound fantastic.

I have Quad VA-One,EL84 control amplifier @ rated 15wpc.8/4ohms...My room is 16' x 14'x 8' & acoustically treated...Last week I demo'd Fritz Carbon 7SE & they were almost perfect..Except for what to me was a very slight darkness to the tone,like a thin veil(think Sennheiser HD650) they were everything I was looking for...
 Yesterday I set up an unbroken in pair of Harbeth M30.1..Granted it's only been a day but I think my search is OVER!
 FORGET all the reviews that say they have no bass & do NOT come alive without power..In my room they absolutely sing with the little EL84 amp!Bass is superb!Deep as I need or want & beautifully articulated.Kick drum has real substance..Tonal colors are just unreal,you can almost taste the rosin from a hard bow strike,feel the skin on a Tom stretch...
 IF your room is close to mine in size & you do NOT listen at headbanging levels I would say get a pair of the new Harbeth 30.2 Anniversaries & bliss out!