dac and streamer or seperates

Definitely separates, but you can always start with a great streamer first and see if you are happy with your current DAC.....if not then upgrade. I a higher end streamer will definitely impress though

Moors Law is something to consider:  DAC/Streamers are changing for the better very rapidly. As such they are depreciating rapidly due to almost instant obsolesce. Think flat screen TVs. 

If being smart with your money matters at this point in your HiFi journey you would do well to buy "pretty good used" and wait for the tech and prices to move in your favor. 1 year from now your $5K new unit (s) will be worth 1/4 of what you paid and the new stuff's sound and features will blow the one year old stuff away.

Now, if you have money to burn buy the best DAC and streamer you can find and understand that it will not hold value and worse it will be rapidly obsolete relative to the new stuff's sonic quality and features. 

I think it's manifestly clear that the DAC/Streamer/Preamp future is going to be Chinese all-in-one solid state and controlled from your phone. Maybe a few US manufacturers like PS Audio or ARC will have a competing version that "feels American" which I personally would prefer as I like to spend my money locally. 

I have a HiFi buddy who's an MD with cash a-plenty. He idled his ARC REF5 preamp and plays his music from his PC into a chifi DAC with balanced outputs into his tube and SS tri-amped towers. I have to admit it sounds surprisingly good with the 256 bit MP3s he uses. I would never have believed it had I not heard it myself. DACs can digitally emulate that LP sound now too. 

Separates for sure. Magic is in the DAC+.  At some personal risk for blasphemy, look at the WiiM Pro streamer @ $150 and a separate linear power supply.  If you believe in measurements, and in this case I do, check out VintageFlanker’s review on audiosciencereview.com. Literally perfect digital output via optical or coax. Don’t need the plus, since not using onboard DAC. Great software app interface. Runs all the best streaming services native, no chromecast or airplay needed, both suck IMO. 
 

Then you can splurge on a great DAC + Clock + Ethernet switch + interconnects, hardwire in via isolated switch output, and you’ll get great organic sound with superb imaging. R2R look at Denafrips, lots of other great options like Chord. 
 

Happy listening!

OP, just keep in mind your system when choosing a dac. The Krell is apparently sensitive to what it’s paired with, based on the reviews I read. Very powerful unit. Also, B&W speakers have a certain sound. Do you want a dac like a Denafrips, r2r, or tube to add a bit more relaxed and natural sound? Or do you want something a bit more analytical? Think of what you currently have and where you’d like to be. How much money you spend is certainly important but what’s more important is not wasting money. I’ve made those costly mistakes because I wasn’t sure what I wanted and just tried things. Take your time and be honest with yourself about the sound that your ears and heart love. Good luck

I would buy a good DAC. Streamer tech is changing fast these days. Even the very cheap ifi zen stream ($400) sounds real nice.