Upgrade Path - Preamp or DAC?


I need some advice on what to upgrade next - preamp or DAC. I don’t currently have a DAC. This is for my second system, which may or may not end up being my primary system. I will be streaming Tidal via Heos on both systems.

My second system is:
Rotel RC-980BX
B&K ST2140
Paradigm Monitor 9

The weak link is the pre. The Heos has analog outputs so I could use it as a DAC and send straight to my pre. The Rotel pre has all analog inputs. So I could upgrade to a pre that has a better DAC than the Heos and optical/coax inputs. Or I could keep the pre for now and get a better DAC. I am lusting after the Cambridge Audio 851D on agon right now. The third option is to just get the Heos now and see how it goes before upgrading. I really want that 851D though, but have just spent a lot on recent upgrades to system #1.

Thanks.

Ken
treebeard1
I have no experience with that cartridge, however many on this forum have spoken highly of it within it's price class. Not knowing anything else regarding your vinyl system, considering this cartridge, I believe the phonostage within the Rotel pre to be a weak link.  That said, if you found a line level preamp you liked, you could still could use the phonostage within the Rotel until you identify (afford) a outboard one.

Regardless of system expense, balance is important, a single exceptional component will not be truly appreciated, one component as a weak link sets the standard throughout. 
I had not thought of keeping the Rotel as a phono pre but that's a good idea.  The path of least resistance is to buy a used phono pre and keep the Rotel for digital until I can spring for a better pre.  I will wait to see how everything sounds once I'm streaming Tidal.
Seems we are both up early. A step at a time is prudent. If where I live I had better internet connection I also would try Tidal. 
I am going to get a new Oppo disc player and use the DAC in it for a while.