I would have agreed if the headphone mentioned where perfect, but the HD800 is a clinical, somewhat bright and peaky sounding headphone in desperate need for something to fix these obvious flaws. It sounds like we agree to some point on this. The point is that no headphone is perfect and whatever means you have to reach closer to what you consider perfection is the right step forward even though it means coloring a flawed headphone.
What these headphones does best is imaging and here I feel the Leben outperform the M1HPA by a margin. I have only come across tube or hybrid amps that have that holographic ability. Some say B22 does this good as well.
If the Leben + HD 800 satisfies you, great. Sound is very personal.
But myself, I hear too much coloration in the bass and too much loss of finesse with the Leben and HD 800 (especially with bass boost on). There is a strong mid-bass boost that just doesn't sound good to me. It's like vinyl vs digital, smooth and groovy but not clean enough for me.
While the Leben is synergestically good with the HD 800 you can get significantly cleaner sounding amps for much less, and if you happen to own an LCD-2 as well it won't go well with the Leben, it will sound too dark and thick. LCD-2 still has superior bass and midrange naturalness than the HD 800 + Leben IMO. So unless you're heavily addicted to the HD 800, the Leben isn't a good solution...
Finding a match that checks all boxes with the HD 800 is not easy, I'm considering selling mine and buy an LCD-3, but it's such a pretty headphone (darn those germans) and just for curiosity I'd like to keep it just to see if it grows with every system upgrade, as people claims