No, pre-amps are only needed for phono level devices such as older turntables. I assume your DAC is a line level device.
As a guess (and this is an odd setup, so merely a guess), there’s something odd going on with your DAC. As a test, I’d try a different output device connected to your PLAY:5’s line in, and then play a bit with the line-in’s sensitivity settings, before going back to trying the DAC. You’re wanting to eliminate possible variables, right now, the easiest one to check is the line in on the PLAY:5 (or do you have a Five, which is the successor to the PLAY:5 gen 2, there is no 5). You also might want to confirm it is a DAC, and not an ADC device.
Which Blu-ray player are you connecting to? I’ve never heard of one without a right and left RCA output (not saying it isn’t possible, just haven’t heard of it before).