5870 no boot screen via digital

I have 2 monitor system. To date it was configured like that. One monitor was connected to the genuine 'mac upgrade' 5870 card (ie not a flashed or plain pc one) via HDMI, @1920x1080; the second one was connected to the card with a DVI to VGA connector and then to an analog flat screen @1680x1050. The analog monitor was ana Acer one, the digital, HDMI only - Samsung. In the setup and the connection scheme all was as expected: the analog monitor showed the boot screen so I could launch the startup disk menu during boot. The only glitch was the boot screen resolution was not the expected 1680x1050 but some other (I guess something 4:3 because of the image shape). Anyway, all was ok. Because I've bought a second digital Samsung I connected it at first via HDMI. Now There was no boot screen on either monitor. Then I chose to connect the monitor via a DVI to HDMI cable (the new monitor is also HDMI only, no DVI connector) with the same result - no boot screen. So I decided to connect the monitor via the VGA connector (it has 2 HDMI, one VGA) and at the computer side it is the same DVI to VGA interface used previously. Now the boot screens appeared but again, not the ntive resolution (ie not 1920x1080 for the new monitor). I've tried some woraround solutions to achieve the desired result for no evil. Ie: reseting PRAM, changing resoultion to a non native reboot, changing back to the native.I even booted the system in safe mode (shift+boot), changed the resolution and back - the same result. Because I have a multiple OSX installed I intentionaly didn't write what system I was testing it on because in fact, it doesn't matter it is 10.8.5, 10.10.5, 10.11.1 or even Win10 - everywhere the same result.

What did I miss?