I have read really great things about the Leben amps. Some people who liked the Millenia with their speaker systems got great results using a tube preamp, to get more of the kind of sound they enjoy. I have used tube equipment with my various speaker systems, on and off for many, many years. Some gave me great sound...... there can be something very special with well designed tube gear that is hard to duplicate with solid state. I have an audiophile friend who bemoans dealing with his tube equipment, but I don't think he would ever be convinced of going solid state.
I assume you use the headphone out?
I don't. I found that the headphone out did not shake the HE-6s well enough. I started using the speaker output using the resistor box but now I use only two 10ohm resistors in parallel to not attenuate the signal and get the maximum possible power of the Leben which is not that much for the HE-6s.