Where to compare dof between the 6D and 1D4?

ISO is simply a matter of processing.  The only effect it has on noise is less read noise at higher ISOs (e.g. ISO 1600 is less noisy than ISO 100 pushed four stops for a given scene, f-ratio, and shutter speed).

So, I repeat:  for the same scene, framing, shutter speed, and DOF, the photo will be made from the same total amount of light, regardless of system (or ISO setting), so any differences in noise will be due entirely due to differences in sensor efficiency.

For example, if we take a pic of a scene at 50mm f/1.4 1/100 ISO 400 on crop and 80mm f/2.2 1/100 ISO 1000 on FF (same perspective, framing, DOF, and shutter speed), the same total amount of light will fall on the crop sensor and FF sensor.

On the other hand, if we take a pic of a scene using 50mm f/1.4 1/100 ISO 400 with both cameras, crop the FF photo to the same framing as the photo from the crop camera, once again, the photos will be made from the same total amount of light.

In neither case does ISO have anything to do with the noise in the photo, except inasmuch as the higher ISO setting lowers the read noise (increases the efficiency) of the FF sensor.

The FF sensor will always hold the IQ advantage so long as it is not cropping to the same framing as the cropped sensor camera, the sensor is at least as efficient, it has at least the same number of pixels, and the lens on the crop camera is not more than 1.6x as sharp as the lens on the FF camera.

Well, that day has not yet come.

The "attractive shallower DOF" is subjective.  In any case, sure, if you put a huge 600 / 4L IS on FF, it's gonna give you the IQ and shallow DOF advantage over a 400 / 5.6L on crop.  What's that weigh and cost again?

I forget -- what's the difference in size, weight, and price between the 300 / 2.8L IS and 400 / 2.8L IS?  And no, IQ and the final result are not always, or even usually, all that matters.

Because sometimes DOF matters more than noise.  How often "sometimes" comes up depends on the photographer.  I mean, why do I have to explain this to you?

No, it doesn't.

Who freakin' cares?!  I mean, seriously.  Use 24mm on FF, and 15mm on 1.6x.  For example, the landscape shooter using crop could use the outstanding Samyang 14 / 2.8, and there are any number of outstanding UWA zooms for crop, not to mention the outstanding 7-14 / 4 for mFT.

Now, for ultra shallow DOF with UWA (a love of mine), FF has no equal in smaller formats, and never will.  It's one of the reasons I shoot FF.  But who denies this?

If you can afford every lens and camera body made, and a crew to carry them around for you, then you have more flexibility still.  So?

You just seem completely incapable of understanding that not everyone has an interest in ultra shallow DOF, and they don't want the size, weight, and expense of FF for deeper DOF photography.

But it doesn't have the same pixel density, and it's still larger, heavier, and more expensive.  What part of that do you not get?