How to divide up your budget in building a 2-Channel HiFi system

Just a question or two.

What is the size of the room you're working with?

Have you looked at room treatment, nothing fancy, just effective.
I use the absolute minimum I can. NONE if possible. As long as the sweet spot is great, and the neighbors or partners aren't complaining..
That's good.

Is something missing from your system? Sound wise. Is it what you want to hear?

Are there any unwanted noises, hums, buzzes, yoyo sound, any noise when your idling? 

I listen, and listen and listen.. Move a speaker a 1/2 inch. Then listen and listen....  Parts is parts, we use to say. How you use them, or not, is really what the "Tweak" is all about.. To decouple or not. (I always do), Vibration control (Vinyl especially).

Separate the wires, no touching, cable risers.

I know I didn't offer a budget guideline, but these things will add a lot of quality at almost no expense.. 2% of your budget, but 20% of the SQ. 

Get the sound right, then add all the sources you like.

I like them all, I like music, just no Yoko Ono...

Regards...

Mv, I think it depends on the way you plan on approaching this endeavor. 
Some people want to buy a system and get it over with. But, I think most of us plan on approaching this in an evolutionary fashion building a system over time as finances allow. Since you seem to be avoiding vinyl and you already have a quorum of electronics you should most definitely focus on speakers. Take your time. Many people can not overcome their instant gratification impulse. Listen around. Go to shows. Get an idea of what you like best. In the mean while stick all your spare change in the cookie jar. Do it right and do it once. Once you have your dream speakers then you can modify everything else to fit including room acoustics. 
Speakers are too imperfect for me to make a recommendation. It comes down to personal taste and other issue like size and appearance enter in.

As a fun exercise and assuming you had a $20k budget how would you break that up in building your system? Goal would be to optimize realism and expanded sound stage. 

If that really is the goal then why did you put the irrelevant pre-amp/amp, DAC and streamer restrictions on there? That's not a system you're building then, its a component shopping list. 

If you really do want "to optimize realism and expanded soundstage" then here's how you do it. Nothing is any more important than anything else. So you allocate equal amounts to everything. Only you have to do it right, by general function not a shopping list. The whole point of the budget is to help you come up with the shopping list. Don't put the cart before the horse.

So what do you need?

Source, amp, speakers, wire. Four things. 25% or $5k each.

Now if you want a preamp and amp go for it. If you want a DAC and streamer go for it. Just know you have $5k total - and that the extra interconnect and power cord have to either come out of that budget or reduce the amount you have to spend on the others. Because your entire wire budget is the same $5k and that has to cover speaker cables, interconnects, and power cords for the whole system. Conditioner too, if you decide one makes sense.

Oh and by the way, if you wise up and realize you want a turntable that's a source. Arm and cartridge, source. The phono stage is an amp. The power cord(s) and interconnect(s) are wire. Keep your budget straight.

One more thing. This is just the simplest most dumbed down version of how to do it. The more sophisticated approach is to realize there's a whole lot more contributing to "realism and expanded sound stage" than those major components. There's also room treatments and tweaks. $20k is well into the range you could easily throw all those in there. This would shift the budget around to five categories, Source, Speakers, Amps, Wire, Tweaks. $4k each.

While staying within budget, I mean. Which was the whole idea. "How to divide up" thats a budget. Right? 

Mv, I think it depends on the way you plan on approaching this endeavor.

Exactly. The budgeting I just did is for "one and done". Inevitably everyone winds up somewhere down the line wanting more. The smart thing is to see this coming early on. Then if you find something you really love, or come across a killer deal, or anything that seems worth the stretch long term, then of course you can go for that and consider it an "anchor" or Grail piece. But that is Budgeting 201. I thought we were doing Budgeting 101 for fun.

This thread may be of interest:
https://forum.audiogon.com/discussions/the-system-percentages-thread

Everything I've learned in four rules:

1.  Buy used if you're not sure. Mistakes are more forgiving.
2.  Buy quality analogue; it will last a very long time.  
3.  Don't overspend on digital; it will become outdated quickly.
4.  Admire, but don't covet thy neighbor's toys.

Cheers!

Ask your local dealer now!!

Go listen to several systems. Get to know yourself regarding listening preferences.Get to know how components play into your room.  I believe your current system retailed at ~$6500-$7000. Given that, yes you could achieve a significant improvement at $20,000. 

I believe your Tubadour DAC would be at home in a $20,000 system. Have it upgraded to the SE model, if not already there.  Explore speaker/amplifier combinations.