First a response to Keith. Thanks for your follow-up, as you had me questioning my sanity when I read your workflow and tried to understand and replicate it.
A couple of other points. I couldn't see a lot of change in your images when I moused over them, other than the check boxes and other settings, so it was hard to follow what you were trying to achieve. This may say something about my browser.
There are a lot of profile conversions in your workflow, and several of them are unnecessary. If you use the QTR-Create-ICC application, rather than the QTR-Create-ICC-
RGB one, this will create a monochrome ICC. You can then simply
assign to the ICC, and
convert back to gray gamma 2.2. This is a lot simpler. I'm not convinced that this is going to result in a linear workflow, but I'd need to do some measurements to be certain. It may.
So which of the two soft proofing methods discussed is better?
A) Select Preserve Numbers box, or
B) Assign to Profile then Convert to Profile
I would say (B) because it's less editing work to make your Assigned copy look like your master copy than it is to make your completely unmanaged copy (numbers preserved) look like your master copy. (The master copy being the goal or target version.) The Preserve Numbers trick gives you a farther distance to climb, while the Assign/Convert trick gives you a head start.
There are similarities and differences. To repeat, if you are printing to ABW using printer manages colors, then a preserve numbers soft-proof will be a reliable one, as it reflects what you are actually doing.
Taking my amended version of Keith's workflow, if the image is in gray gamma 2.2 and you
assign it to the (monochrome) ICC, then that is much the same thing. It's a soft-proof of your workflow. Photoshop is showing the image as it should appear if printed as-is without any color management. If you look at the monitor in most cases the image will appear different (not much different for certain PK papers), but the histogram will be unchanged. What you are doing by assigning the image to the ICC is instructing Photoshop to interpret the numbers differently.
Open the image and duplicate it and preserve numbers soft proof one and assign the other and they should appear much the same. The main difference to soft-proofing is that there isn't the option to simulate the black ink and paper color. If you leave those options unchecked the two images should be identical or nearly so. If you want those options used to proof your assigned image, you'll have to do the soft-proof as well.
So far so good. If you continue to follow the amended Keith workflow and now
convert it back to gray gamma 2.2, the appearance on the monitor won't change (you might lose a color tint), but the histogram will change, in most cases. This is not soft-proofing any more, you've changed the image. You can see this from the change in the histogram. The reason that the monitor doesn't change is that you have also instructed Photoshop to interpret those numbers in the file differently, so as to leave the appearance on the monitor unchanged.
This is an image editing technique, not a soft-proofing one. Because you've changed the data in the image, it's going to print differently using your workflow, which makes no allowance for the image's ICC. If you want to get an accurate soft-proof you're going to to have to use the preserve numbers soft proof. You may find this image editing technique useful, because in a lot of cases it's going to lighten the shadows - as you will see from the change in the histogram How much will depend on the image and paper. But to simulate the impact in the print, and assess what further edits if any are required, you'll have to do the preserve numbers soft proof, because you are printing without reference to the ICC.
(The various platine papers, genuine and rebadged, are one exception where you won't see this effect very much.)
Everything I've said has been with reference to printing via ABW and printer manages colors, but the exact same logic and procedures apply to printing via QTR without using the ICC to print. I prefer the QTR prints. And there's more scope to tweak.