Digital/Optical out from Streaming Devices (Bluesound, Heos, etc) to Preamp- difference?

It all depends on how high a resolution system you connect it to. There are audible differences between the digital streamers. IMHO, most of Mapman's comments are spot on; dac differences will be bigger. However, once you settle on a dac, improvements in streaming device will be audible on a good rig. If you never plan on moving beyond an entry level system, it may not matter too much, but in this hobby, the devil is in the details, and they all matter.
I'd suggest trying to demo the devices on a higher end system in addition to your own to get an idea of the how those devices compare. If it doesn't sound like much difference to you, save your money. 
BTW, suggest adding Sonore SonicOrbiter SE and Aurelic Aries Mini to your consideration list. Cheers,
Spencer

" the devil is in the details, and they all matter. "

Right but in this case what matters is Jitter which can be measured with the right tools and know how. You expect better performance including jitter with a more expensive device but that is not assured in any particular case by price alone.

There is a threshold for jitter that is generally deemed sufficient below which differences will not be heard.

My experience has been it is very hard to hear the differences once you achieve a certain level and digital has improved in recent years to the point where most good quality source devices achieve very good performance.

No doubt the less jitter the better. The only question is how much is needed or matters and how hard is it to get that. Not very these days I would say.

Maybe there are independently verified device specific jitter measurements published somewhere that might help. Other than that it is mostly a pot shot.

There are recclocking devices that can be added as an insurance policy against jitter if concerned.  I have considered those but not been tempted to jump.   Would love to try and see.    Also many modern DACs the most well known example being Benchmark do their own reclocking and are also highly jitter resistant as well.   The thing there is you get low jitter tied to a specific DAC and sound which may or may not float one's boat.    With  a separate reclocker, you have an insurance policy that can be used with most any DAC.

These days I have identified multiple digital sources all capable of "high end" performance. These include Roku Soundbridge, Squeezebox Touch, 8 year old Denon CD player/recorder, USB out from Gateway Windows laptop, optical toslink out from Amazon Fire TV box, and analog out from newer Apple Iphone 6s and tablet.

I was pleasantly surprised by sound quality of USB out from the laptop and analog out from newer Apple devices, but each of those do sound different yet all still very good and "high end"  when playing through a good "high end" system..