Upgrade Hint 6?

It's hard to say, really.  I'm assuming you'd connect the outboard dac into the Hint and hope for an improvement?  That would depend on if your speakers are capable of presenting additional detail and resolution.  Also, the Hint is said to be on the slightly warm side soundwise, so a slightly warm sounding Bifrost or Yggy might push things too far into the warm category to hear an improvement.

Then there's the cables.  The DAC in the Hint is pretty respectable, I'd say you're at a point where you'll get diminishing returns.  As long as your speakers are up to snuff, I'd say just call it a day and stop researching new purchases.  If you are bound and determined to get something else, you'll get more differences from getting a second pair of speakers that you can swap in when you get bored of your sound.  

You'd be much better off selling your Hint and putting your $$$$ toward separates or a better integrated. That or just enjoy what you have.

DACs really don't get any better than what can currently be bought for ~$400. The technology peaked years ago and what's available now are basically two types: highly-accurate, near-perfect-measuring DACs (that can be had for a couple hundred), and DACs that color the sound in some subjectively pleasing way, with rolled-off highs, distortion, poor channel separation, etc. 

The Schiit Multibit DACs fall into the latter category - colored sound. 

OP, don’t be surprised to find disagreement among audio people.  Calling Schiit multibit DACs colored sound is not true.  And as one who’s owned three of them I think I know a thing or two about their sound.

Sorry, but I’ll also have to disagree with the $400 dac theory as well.  But one does need decent gear in order to hear improvements.  Good luck with your search.

"DACs really don't get any better than what can currently be bought for ~$400. "

I didn't know that.