System Upgrade

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)

P.S. to my previous post:  Unless the manual or the manufacturer or dealer specifically indicates that it would be ok, I would not change the setting of the impedance switch on the rear of the amp while it is powered up.

Regards,
-- Al
 

Sounds good but "flat"...

Coincidental word game, but Leben website proudly claims...

"FLAT and wide frequency response"

How old is that model? They also say it is "capable for future digital audio format such as SACD/HDCD."

https://lebenhifi.com/products/cs600.html

More power will make a huge difference, IMO.  I use a 300 watt amp with the SHL5+.  Flat it is not.