Need suggestions on efficient book shelf speakers for a Pass Labs XA3.5

Fritz Carrera Be or one of his other models.

Your amp puts out close to 130 watts when driven out of class A.  That's sufficient for nearly all bookshelf type loudspeaker at reasonable levels.

@puffbojie ha! I drive Wilsons Audio Sabrina with XA30.8. Your XA30.5 is 30w into 8ohms in pure Class A at lowest distortion rated by Pass Labs. It then switches to Class A/B and delivers around 130w/ch. You know it’s in Class A/B when the needle on a bias meter starts to move. 
 

What’s the rest of your set up - source, preamp, running balanced or single ended? How could the volume be too low? Define low volume - knob position or step number relative to max setting. 

I had a 150 watt class d amp previously and remember it being much louder at an 8/10 volume (using same speakers) .  The amp is at a pleasant volume as it stands but it seems capped to my ears -volume wise. I have a Benchmark 2 Pre/Dac and a Bluesound streamer using rca interconnects.  

@puffbojie Benchmark DAC 2 with built in pre? I would seriously consider a good linestage to go with the XA30.5. I just sold my Benchmark DAC3 HGC that I tried as a preamp for few days (it was sold due to a DAC upgrade and not because preamp in it wasn’t up to the task).
While it was accurate, it was no match to my Pass XP-12 when it comes to dimensionality, soundstage and natural tone. Also, in my experience Pass amps sound best when run using balanced interconnects. Going from benchmark into pass balanced should result in higher gain and lower noise floor. Give it a shot.