Amfibius,
I wouldn't call the Avantgarde's "harsh and trebly". The AVG "sound" if you will, is revealing and VERY dynamic. I think whart does a good job describing things... If I can I'll add a little more to help you clear things up.
I came from big Martin Logans (Prodigy's) which were really nice. They threw a huge soundstage and were very fast and revealing..... But, laid back and polite.
Now, with the AVG's things are still VERY fast and VERY revealing. The main diffences are that these speaker are not laid back; I don't mean they are aggressive (although they can be w/the wrong front end or recording). They "jump". They are very dynamic; that drum thwack is gonna thwack man! It took me a little getting used to this "sound" after my nice sweet, polite 'stats...... Now there is no going back. Who know's where the journey will end, but I have to say, this horn thing is really cool.
More....... Take it as gospel that you will hear everything; cell phones, tube changes, the master-mix in the next room if it's on your same circuit, but like other have said; you can get it all sorted out with a little effort.
I'm not so sure about your room; it looks like you have your ProAcs pretty close to the front wall. You're going to need some space behind the AVG's (think 3-4 feet min.).
I'm not so sure about your amps with the AVG's either.... Transformer noise is an issue and you need quiet amps.
Hope all of this helps.
Chris