So this existing system has a Pioneer stereo receiver feeding speaker level to a home made 70Volt stereo/ two channel transformer system.
We're just replacing a few speakers in a remodel and switching out old faded volume controls. I also want to install a Sonos Port new two channel amp instead of the Pioneer.
But the issue I can't find any 70V stereo/dual channel volume controls. The existing volume controls are Atlas #DAT-10-W
I don't have any 70Volt experience so not sure what options are if I don't locate any similar volume controls.

[Link: photos.app.goo.gl] Volume Control

[Link: photos.app.goo.gl] Homemade 70 Volt Box

[Link: photos.app.goo.gl] Homemade box again

SORRY, THE LINKS DON'T WORK I'VE TRIED A FEW TIMES

I can help you out, I think, but I'd like to have actual numbers so I can write about just what's needed. That said, I found dual 70 volt volume controls pretty quickly. This also tells me I need you to tighten up your spec.

Why, I literally mean what is the reason, the logic, to putting the Sonos amp in instead of the Pioneer? What is insufficient or wrong about the Pioneer?

Lastly, do you need or prefer these controls to be 1G wide, or, say, 4 up on a stainless steel plate, or ?

The photo links don't work. I suspect they point to a place in your computer where those photos reside... whatever; they don't point me anywhere helpful.

You are replacing the source.
You are replacing the amp.
You are replacing volume controls.
You are replacing a 'few' speakers.

So in other words, you are completely replacing the whole system except for some of the speakers.

Why! Why would you not go ahead and replace the last few speakers and put a conventional system in?
If you say 'budget' I call BS. You need to educate the customer on why trying to save a few shitty speakers is not going to save them any money. And in fact, trying to Frankenstein another version of this same system is probably going to cost them more.
This just doesn't make any sense. Unless we are missing a lot more details. Like the system has 20 audio zones with 40+ speakers and you were only replacing 4-5 of the speakers. Even then I got to question the condition of 'all' the existing speakers, considering the typical product that would have been used for this and the age (neither of which you mentioned either).

My favorite supplier can offer a 70 volt stereo volume control.

The words "homemade 70Volt Box" would absolutely scare the shit out of me, and I would instantly start looking for a better solution.

“Homemade 70V box”? Why? Is this simply a scheme to match impedance for the Pioneer? Are the speakers just cheap 70V restaurant grade stuff?

Ernie: A Sonos PORT is a line level device.

I can't turn it into a conventional 8 ohm system. The wiring is 18/4 solid thermostat wire daisy chained........

Oh my! Probably, a long time ago, an electrician installed the system based on something seen in a restaurant. I can imagine an original 70V something driving the system. Later, someone “improved” the system by adding matching transformers and the Pioneer.

Are these restaurant grade speakers or something better?

I don’t know your budget constraints, but you could pull out a few rooms using Sonos AMP’s, here’n there. I’ve put Connect:amp’s in closets above ceiling speakers in impossible to wire homes.

Why can't you?
It doesn't matter if the wire was home run or not. What matters is 'how many' speakers are in the run. You can do up to 16 sets of speakers with some impedance matching volume controls before you would need a 2ohm stable amp. You still haven't indicated how many speakers we are looking at in this system.

The other problem is the 18awg wire. For that, we need to know what the total length of the daisy chain is. And then we can figure how much wattage we could put thru it in a low impedance system.

Again details matter. If this system has 20+ speakers in it and/or total wire runs over 200 feet. Then yes, 70V would be the way to go. Still don't necessarily need stereo VC controls however. Could do just conventional 70V mono.

How much of this is being driven by customer demand?

100, 70, 25V, 16, 8, 4, 2-Ohm — it’s all the same physics, just different math. We are simply ensuring that the loading is appropriate for amplifiers, wiring, controls, and speakers. With power wiring there is a single frequency. With audio there is a range of frequencies and the loading varies with frequency. “Impedance” appreciates and reminds us of this variation. (Unfortunately, with some added math)

In some respects we could consider our 8-Ohm stuff as a 25V system.

This last sentence echoes my comment: you have to tell us just exactly how many speakers you're trying to drive.

It also points out that you need to know how to determine the load that each speaker puts on the amplifier. For that you need some almost basic math.