Well, the $400 could put me on the audio road towards a Schiit Bifrost, even a Gungnir...or one of the Audio-GD units (not decided yet)...a significant leap up from my current DAC... Mmmm...DAC or LPS-1 ? ....might have to toss a coin !
My thinking was that you've got a $250 DAC/amp, why go to a $600 one? I don't have a lot of faith in the miracle $600 DAC. You've got a great USB source, so why don't you hold out on the intermediate steps, wait a year to save up if you have to, and go into the $1000-2000 DAC range next?
I'm just saying, the intermediate steps aren't worth it and you'll be happier and save more money if you hold out for a real component. If real for you, and you've already heard it to confirm it's no longer worth the cost-to-performance increase, stops at a $600 DAC, then go for it. I'm not saying you're wrong, I just wanted to say I'd get the PSU for the mRendu and be happy that component is completely taken care of, then invest more heavily when I was really ready for a true DAC upgrade vs. a marginal one.
In saying that, if the mRendu is going to be your source of choice, then USB input will be your default input of choice. That means whatever you are looking at better have a good USB input or you'll be losing out and need to get one of the USB fixers or USB->AES or USB->I2s convertors. I'm saying this because you mentioned the Audio-gd units and they're not famous for having a killer USB input. The newer ones with improved Amenero input boards are decent, but still, the people that really like those DACs are using the I2s input... And the upper range Schiit DACs are using the AES input. So, have a good read in this for whatever you do~
Any way you do it, it's all good. Enjoy your gear and the music