Hi Daro,

An MST Hub is an active device so you cna use Passive/normal cables to connect your screens to the MST Hub. Which Card do you have, because i do not know many R9 390’s that have 2 DP outputs …

thanks for the reply,It has only 1 output dp , connect each other by dvi-hdmi.

My problem is that they do not look the same ,which is connected by dp looks darker .why?

sorry but I just 4 screens and one by dp, the other by dvi-hdmi

Hi Daro,
Not really following what you are saying :-S
Anyway to connect 6 screens to a reference designed R9 390;
Outputs will be 1x DP + 1 x HDMI1.4 + 2 x DVI-D. 
Normally a R9 390 can output “natively” or “directly from the graphics card” a total of 4 screens.
If you want to conenct more, you can up to 6, but will need an Displayport MST Hub.
First you can use standard cables to connect tp the HDmi and 2 x DVI-D ports of the card, and then connect a CSV-5300H to the Displayport output and connect with normla HDMI cables to the screens. I hope that answers your question.

that's exactly my configuration, works perfectly but the  screen by dp looks different because it can be ?

well there can always be minor differences beteween the screen itself and when converting a signal, however when signals remain digital (DVI-HDMI/DP) those difference should not be big. 
Are all your screens the same Brand/Type ? Does it help when you switch screen from DP to HDMI or does the one connected to DP still give darker picture ?