Power - where to start? Mains, chords, conditioner, filter?

The biggest improvement by far will be to replace all those crappy factory power cords. Literally anything at any price, you could throw a dart, will be a big upgrade. Don't do that however! 

Instead, go and listen. Ideally bring as many home as you can and hear how they sound in your system. Not because they are system dependent but because you know what your system sounds like. Or if you go to a store then at least bring one of your crap cords along to compare. Prepare to be shocked! ANYTHING at ANY price will blow away what you are using now.

BUT here's the thing- if you really do try a lot of them you will eventually find the really good ones will blow away the rest even many times much more expensive ones! The gold standard, not because its the very best but because its been around so long and is crazy good for the money, is the Synergistic Research Master Coupler. They've been making this for well over TWENTY YEARS now and there's a reason- its that good! So good they sell used now for more than mine cost new. Not saying you can't find better. Saying you owe it to yourself to check it out.

That's by far the most important advice I can give- go and listen. Forget construction. Forget reviews. Forget everything anyone here recommends- including me. Just go and listen.

Where to start is to consider the entire power flow set up by thinking of it as an entire system. I first got a power conditioner but it wasn’t until the wall plug, cords et all was changed that the sound morphed like it was top end system that’s a on a medium priced system (compared to many).

Despite having a great system it was lagging/flat and after measuring (scientist friend) here at the house I found out that I have poor dirty power. I did everything I could and the ultimate critic who could care less (my wife) even says wow..... and now she put’s on music..ok then...the fix was all in a bit over 2k. I did months of research. The buzz term is to make a quite noise floor.

I found on ebay LASPADA AUDIO APOLLO II 10 and 12 gauge Rhodium plated power cables...(Wow!)..very reasonable for what they are.

They in turn go into a used Furman it reference 15i power conditioner (more wow also consider AudioQuest Niagara 1000 used on ebay or the top 5000 ) and that plugs into a

Furutech GTX-D NCF(R) Ultimate Nano-Crystal Rhodium-Plated AC Duplex Receptacle with wall frame and cover... maybe overkill but an upgraded plug is key. You can read up on this and what to get until your eyes pop out...But all agree it’s a must.

House can’t be torn apart to put in dedicated 20AMP lines so none for me. (Or I would have gotten a 20AMP power conditioner...make sure to get the right amperage as 20 Amp plugs are different)

Also added a Synergistic Research Blue Quantum Fuse (who would have though that could matter)

as well as a few PS Audio - Noise Harvesters

If your gear is tight on shelves you will have to allow for more room as the upgraded cords require a more gradual bend as they are stiff and large.

Would it make sense to spend less on power cables and rather put that money into one of the PS Audio Regenerator? I see that a more affordable one has recently been released, the Stellar Powerplant 3 @ $2199

There is some common thinking here among the answers, and I agree - start as close to the beginning of the chain as possible, then work your way down.  At the panel/in the wall if you can, then outlet(s).

If these upgrades are not possible/feasible, the thinking about starting at the beginning is still the same.  Start with the cable powering your line conditioner (or start with actually getting a line conditioner if you don't already have one), then your source(s) - streamer and DAC in your case.
It may seem counter-intuitive to upgrade power cords on lower power draw components before your amp, but if/when you do, the "goodness" filters through the whole system, IE. you will be amplifying a better signal/source.
Good luck!

One of the best investment for me was a PS Audio Dectet https://www.psaudio.com/dectet-power-center/