The Hi-Fi Rose RS-250 is a much smaller unit in physical size. This translates to smaller power supply, less circuitry, no modular board sections, etc. That is the primary price difference.
I do agree that an all-in-one probably won't perform as well as a really good off-board DAC combined with your Rune Nucleus + 100W HDPlex. There are some servers that have good DACs built in, but they are generall over $10k.