I've already answered and they now got back to me asking for some audio files and the Reaper session so they can look into it, which is what I was expecting in the first place (and not some message to switch to A5).
I'm not an expert on Reaper but tried to look into all routing settings and didn't find anything wrong. In Scarlett I'm not sure but my interface (Scarlett Solo) is quite basic and except for the "direct monitor" setting can't see other problem there.
What makes me think the problem is from Amplitube is precisely because I've tried using Bias Fx2 instead of A4 and the reverb issue disappears. What is weird is that I was using A4 for several months and always worked perfectly... so can't understand why I now get this huge reverb sound either on live input and on playback. I'm really hoping Support can understand where the problem is coming from because I really like Amplitube's quality and versatility.