Understanding the preamp/dac with integrated amplifier setup

In my years of researching, buying, setting up and enjoying audio systems, on many occasions I've read reviews where the reviewer purposely uses a pre/dac with an integrated amp. Typically the reviewer mentions that they turned the volume of the integrated amp all the up as to effectively pass the full signal from the inputs to the amplifier section with little to no resistance. Those are my words\understanding.

I've read this setup in very prominent and established audiophile magazines. Recently a reviewer used the Mola Mola pre/dac with the elekit 300b. In other, the Parasound Hint 6 was used in the same way. In none of these cases is the reviewer reviewing the integrated in this way but rather using the integrated in this way when reviewing another product.

In both of the cases above, the results where outstanding. I've also gone as far as to ask a prominent amplifier designer in the boutique space who commented that this can work as long as you have a good preamp.

My question(s) - is this safe as long as you're truly using a preamp into the input(s) of the integrated and is there a rule\guide for when this could be a good setup?

I'm asking because I have 2 pre\dacs and also a couple of integrateds...why not explore?

Thanks for any information one can provide.