Thanks very much for all those usefull infos.
I'll also see a sound engineer at the end of this month and talk to him. I'll report if there is anything that has not been mentionned here.
Cheers.
Your sound engineer is probably going to tell you how important room acoustics are and the importance of speaker placement relatively to your listening position. The good thing about small monitor speakers (like the A3x) is that they are intended for short listening distances, thus minimizing room reflections relative to the direct output. It is essential that you sit in an exact triangle where the distance between the speakers is exactly the same as between each speaker and your respective left or right ear. Additionally, the lower part of the tweeter dome should be exactly at ear's heigth.
Personally I always use headphones next to my monitor speakers for really finicky editing decisions. Ideally you would like to avoid the smallest noises surrounding your effects, or the wrong traces of breath when cutting dialogue. A good headphone is a bit like pixel-peeping, but it will influence the end result tremendously because your mix will be so much cleaner at an almost subconscious level. When I need to check the quality of complex sound tracks I pull out my SPL Phonitor headphone amplifier, which has the ability to switch phase, mono-left, mono-right, and set different virtual speaker widths. It is a beast of a machine, so I do not want it on my desk all the time. For daily editing use I employ a Lake People G100, which is a fantastic headphone amplifier (from the same people that make the Violectric DAC V800), although without the tricks of the Phonitor.
My editing room is well treated with absorbers and diffusers in order to get an even and well controlled reflective behaviour. When talking to the engineer make sure that he knows the specific needs of film editors, contrary to those of music editors, or he may give you some well-meant but bad advice.