Thanks Mr B and Johnim. Here is where I'm standing, so far with my Samsung LCD TV.
1) DVI+VGA adapter : SuperPlus 800x600 works but picture is not only poor, it's just unusable. Uncentred, with many vertical stripes, terrible.
... Didn't try other monitors like Xtreme or HighGFX... will do but so far Indi AGA mk2 is even worse than a simple SCART plugged on the RGB direct output of the Amiga (apart from the slight flickering wave occuring every 10 seconds and captured on the close-up below) :
2) HDMI: no signal at all.
I was about to be very disapointed when I decided to test on my small NEC multisync LCD 1550M that I just bought in USA, and I must admit the picture I get is very very neat with DVI+VGA adapter.
Problem is how frustrating not to be able yet to have the neat installation that I want on a modern little LCD TV (bought on purpose!), with A600+IndiECS on VGA input and A1200+IndiAGAmk2 on HDMI input.
Conclusion : like a lot of people, I'll have to wait for the new core + configuration software... and hope !
But so far I have a great waiting solution with both the A600 and the A1200 connected with a VGA switch on the NEC. Only drawback is I have to launch an automatic centring adjustment each time I switch between the two machines (but it's quick). Not too bad in the end.
Off topic but here is a picture of the A600 with IndiECS running 256 colours with latest drivers