I know, it was more a question out of curiosity 

Question for who ?

Maybe my impressions will change after more days of use but thus far it seems this HE-9 is an amp I could live with forever.  ..

Some change indeed.  The result is still good. Very good but it seems I hear something a bit synthetic in the overall Tone. My Sonett is obviously a bit more bloomy but there's something more refined, more natural , less synthetic.

I first thought my Sonett was blown away but I'm not so convinced. HE-9 Bass are obviously better , faster , stronger ...

#t=8

and overally the big Audio-GD is faster. What is really good is the mix of power, smoothness ( onctuosité in french  : unctuosity ? ) , and speed with a great soundstage for a Solid State amp. But... something remains wrong in the tone or in timbres. it's not brightness but a kind of synthetic edge ? 

Despite the bloom, The sonett  seems to provide a bit more clarity and finesse. THe HE-9 is impressive at the first time but finally I should eat my words and admit that I would maybe not live with it . Maybe, maybe not.. Jury's still out.

Dunno if that makes sense for some of you. Maybe I'm crazy.

Just wanted to know: If I wanted to try something different when funds allow it, what would would be an upgrade from the Valhalla? 2A3 EC, Crimson, Glass, Stratus etc.

Yes

Did anyone have some experience with the Audio-GD Master 9 against other Solid-State contenders and best Tube amps ?

The fact that the AGD9 didn't knock out the Sonnet and is even a lesser amp is af few important areas, it seems the M9 is not a ToTL HD800 amp. Get a BW or Studio for the 800.

Is the BW really ToTL territory for HD800?

How many ToTL solid state amps do you know for the HD800? I know the Ragnarok and BW+PS (power supply), that's it. Neither are quite ToTL SET level if that's what you mean.

Ragnarok has established itself, very little has been written about the BW and i can't find anything about topology.
I assume Marv has been modest about it then?

Yes. The topology is ludicrous, as in Spaceballs Ludicrous.

https://www.youtube.com/watch?v=ygE01sOhzz0