I have the RME and while I really wasn't a fan of it driving my amp direct, it does have several output settings to achieve the correct gain. Still it wasn't a great match with my amp(s)
The RME has a setting called Auto Reference for the volume. It uses a combination of software and hardware to handle volume. There is a table in the manual that references the dBu scale relative to voltage output.
-5db outputs 1v , all the way up to +13dB which is over 3 volts at "0" ref volume