BUILDING THE SYSTEM
As always the purpose of this section is not to build a functional system but rather to showcase what you can expect from the case at hand in terms of interior space.
If you plan on mounting radiators inside the Dark Base Pro 900 Rev.2 you will need to remove both the top and front covers (both are held in place with several clips accessible from the interior).
We also mounted all three drive cages in the tower to have a more complete view of its interior space.
Of course to mount the power supply you will also need to remove the shroud/cover (2 front and 2 rear screws hold it in place).
Depending on the size of the PSU you choose to mount there's also room for two 120/140mm fans at the base of the tower (to install the right fan you may need to remove the dual drive cage).
To mount the PSU you will also need to remove this internal mounting bracket.
Regular ATX sized mainboards leave plenty of room on the mainboard tray and as you can see from the above picture you can also mount an 420mm long radiator at the top (you will need to remove the 5.25" cage however).
At the front there's enough room for a 240/280/360mm radiator up to 35mm thick (unless you remove all drive cages).
Of course let's not forget that you also have enough room for CPU air coolers up to 185mm in height.
As for graphics cards even with the drive cages mounted there's still room for 325mm long ones so the majority out there will have a problem with that (GeForce RTX 2080Ti cards are up to 327mm long but this is one of the few exceptions in the market).