21700 complicates it a bit since the D10 and D25 are 18650 only. Boruit has some 21700 compatible models on Aliexpress though, but not the same form factor and a little more expensive. I have a D10 swapped to a sst40 5000k. Even with the stock driver i still get almost 1000 lumens out of it and good flood/throw.
I just posted in another thread that the HD20 is on sale on Wurkkos’ site for 28.99 or 32.99 with battery. I’m not sure if that meets your definition of cheap, but it’s plenty bright, takes a 21700 or 18650. However it is 5000K and one of the two emitters is the LH351d which you’ve stated you don’t like. I still thought I’d throw it out there since afaik there aren’t a lot of good cheap 21700 headlamps.
I have a couple of the D25S models. SST 40 emitter is nice & White.
I have Nitecores & other brands too.
For the money this one would be hard to beat IMO. I like it a lot. My “go to” headlamp at this point……
The headband is the weakest point, but it is not that bad. It works just fine. 
For the cheapest and brightest 18650 headlamp, I would still go with the Boruit D10 Headlamp. BLF review here: Boruit Headlamp Review D10 (aka. EHL0628) - King of the Budget Headlamps?
For less than $15 (BORUiT D10 XM-L2 LED Headlamp) and includes USB charging and a decent UI, it’s a bargain!
I like the Boruit D10 as well.
I got the one with a neutral tint.
I prefer warm white, but the Boruit D10 neutral white is very nice.
For a headlamp, a fancy UI would make no sense IMHO. When I use one it is to free both hands up while working on something, the more simple the UI the better. D25S or D25L are not going to impress anyone, but they are just super useful for very little cash.
These two suggested headlamps are about the same price-$15. Are there any known outright comparisons
between the Boruit D10 XM-L2 and the Sofirn D25-S dual SST40 , as to which of these two is the better ?
They are in the right price range—Cheap ! Forget the use of 18650 & 21700 batteries , just 18650 is OK
The Sofirn comes with a battery and is a bit more floody, plus battery life will be reduced. If you care about modding, the D25 is far less moddable than the D10. Otherwise both are good.
Oh , the Sofirn is floody. Do you have any idea of how much throw the D10 has ?
Which one has the cooler color ?
No, my main issue is double click goes to strobe. Why? Who needs shortcut to strobe on a headlamp? I would say the majority would highly prefer 2C -> Turbo/High. Makes no sense to have strobe on such an easily accessible shortcut.
Are there anymore suggestions about what cheap & bright headlamps are available and where to buy them 
I’m looking for the 6500K (CW) color—No green tints please. Any zoomies out there ?
I vote for D25 Sofirn. I also don’t like strobe in headlamps. The bottom line is know your UI. I know better than to double click this light so it’s not a problem.
i second the D25. As I stated earlier I have a couple of the D25S. I could not be more pleased. And at the price it is a NOBRAINER to me.
Nice White light… no green.
Comes complete with battery also. What ia not to like??
Go for it….
Oh yeah, as far as being concerned about the UI… that is WASTED worry. It is simple & straight forward. I have NEVER EVEN ONE TIME gone into strobe modeby accident.
It’s not a matter of just knowing UI - it’s _too easy _to accidentally double click. It’s literally 2 clicks. The bottom line is that strobe shouldn’t even be PART of a headlamp’s UI. If you’re in a group, like a hike or camp-site for example, an accidental activation can be extremely disorienting and negatively affect those in your party. That’s all good and fine if you, like you say, ‘know better’, but for me it takes huge marks off the light if you’re concerned with any type of group activity. Maybe this isn’t relevant to OP, maybe it is, but for me it’s a big enough issue to take note of.
Having said that, I still agree that it meets all the criteria that OP asked for. I’m just throwing out my own personal experience.
To each their own, but for me the UI is of ZERO concern in a negative way.
I have two of the D25S models & have NEVER gone into Strobe mode by accident. Not one time.
And even “IF” I did… so what?? It would be for a fraction of a second. “To me”, this is of no concern.
Just my .02 on the matter.
A headlamp especially a small 18650 one will need an efficient buck driver to put out good sustainable lumens, and buck drivers aren’t cheap, sadly. Otherwise you’re going to top out at ~300-400 lumens (constant).
PL47G3 is meant to have buck driver, but it’s 21700 and not out yet.
SP40 looks good to me. Good brightness, USB charging and is a duel light - headlamp and EDC torch and magnetic base
Of the two headlamp suggestions made here , does anyone know which bright light , “D” models of
the Sofirn or Boruit come with the ZOOM feature ?
I’m surprised no one mention Xanes? I found that Xanes has some nifty headlamps. They actually have quite a few selections.
In the aliexpress link , can anyone , please explain the differences between the Boruit D10 XM-L2
headlamps with the one & two LEDs in them ?—I’m obviously somehow missing something.