Almost everyone on this site is going to say that, your internal DAC and a $5 cable is going to sound the same as a $500 Dac and $150 cable.... If there is a difference, it is not one that our ear can distinguish from, if your equipment has a flat eq and is not changing anything about the sound and you play each DAC at the same db level, the sound will be the same...
I find this to be true.. Amps {of the same configuration, obviously tube amps sound different than full class a amps} all sound the same given the same input and same level, preamps sound the same, dac's sound the same, cd players sound the same, cables sound the same, some like some brands over others, but for the most part you will not hear the difference between modern electronics of the same purpose... In other words, spend how ever much money you want, but understand you are buying the name and aesthetics of the more expensive gear because they are going to sound just like the cheap stuff... Speakers make night and day differences, not electronics...
That being said, are you using this system for music only? If its just for music do yourself a favor and shorten up the signal path, instead of spending money on an expensive dac, buy another sub, or buy a music preamp/dac that handles your crossing... We will never tell you not to spend money on your system but spend it on something that is going to be an audible difference.. Doubling up your subs will smooth room response and blend the system better, running a $100 mini dsp or a preamp that lets you tube the crossing will make a big difference... IMO, changing a dac wont on its own create a difference in sound.. But there are many things that will...