System Upgrade

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.  

A hofer you have totally different gear. 

This gentlman has two really viable choices change the speakers or change the amp which then might necessitate other changes.

A pair of Focal  Sopras or Persona 3F are both more efficient have deeper bass and greater dynamics and the Personas would sound fantastic with the rest of his gear they are 92db efficient.

The Leben gear is excellent moving to more exciting speakers will perk up that system.  A 7db gain in efficiency is huge and his leben would be coasting not to mention the brighter sound of the Paradigms would complement his warm sounding components.

Peachtree Nova 300 will make them shine0

What tubes do you use with the  Leben CS600? Its first iterations used EL-34 or 6L6 tubes with a selector switch between eiher type.
Have you tried EL-34 tubes? Though, indeed, 32W is not a perfect match for Harbeths, with EL-34 tubes you would get a warmer and mellower sound.