System Upgrade

You got mud Harbeths are warmly voiced loudspeakers, PS Warm, Cables warm.

Have you considered a cleaner more efficient loudspeaker?

How big is your room? Musical preferences? Do you play loud or moderate.

A less colored more full range more efficient loudspeaker would maxamize the rest of your investment and you would gain on all fronts. 

Can you go with a floorstander?

What part of the country are you?

Dave and Troy
Audio Doctor NJ

The amp is not the problem. I have one and it is holographic with the right setup. I have also owned shl5’s, they can sound very nice but dont seem to take full advantage of the Leben. Cables and source though are the likely problem. What is your room like?

"You got mud?"

A 30 day old Leben 600 will likely go at very close to full price
as they are made to order often times. If you unload the HL5+
you will take a $2k hit. I would get on the Harbeth owners forum
and get an answer straight from god's mouth.

Not sure which PS Audio, but trying different filters, along with the volume control is a thought.

For kicks, do you have a local guitar center?  go pick up one of the inexpensive or a couple of the inexpensive crown pro amps.  See what a crap-ton of power does for them.  You likely won't be keeping the crowns, but GC has a liberal return policy.   Just for kicks, see what some serious cheap, but powerful class D does with them.  Probably hard to find an amp more different than your Leben (seriously) a quick and dirty way to find out if it's an amp/power issue.  

If they still sound flat... think room, or frankly maybe they just aren't the speaker for you. 

(I'm not claiming the crowns will sound good... but it should/will address if a radically different amp changes the sound)