I was wondering what Qtc should I aim for, as I understand i right now the most popular approach is 0.707, but the Audio Technology salesman
said most of their customers prefer 0.5 and he prefers 0.577.
What's your opinion?
It depends on the listening room size, dampening, and speaker placement, as well as personal preference, but much more on the former than most people realize.
The "perfect" Qroom is 1. That is per definition only achieved perfectly with a Qbox=1 in free-field conditions which is defined by a point in space where the distance to any surface is longer than the wave length of the lowest frequency the speaker can produce.
Any surface will have a negative or positive effect on Qroom, depending on whether it dampening (decreasing Q) or reflecting (increasing Q).
I hope that this little piece of theory makes you realize that Qbox is totally dependant on your listening room, and therefore it is nonesense to argue for a "perfect" Qbox. It is certainly true that most listening rooms fits a Qbox between 0.5 (critically dampened) and 0.707 (Butterworth alignment).
In general, the smaller the room and the more dampened the room is, the smaller the Qbox should be. And the larger the room and less dampened the room is, the larger the Qbox should be.