Preference for separate phono stages?

there are excellent
internal phono stages that are well designed and that are difficult to beat with
external ones (for example Shindo and Vacuumstate preamps).
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Add Atma-Sphere to that list.

And Jadis.

Old AR pre-amps with phono are amazing (ie 6C). However, that is from the true old vinyl days. In the 80's & 90's most phono stages were just an afterthought. The phono stage in a cambridge 640 phono is way better than the internal phono stage on a $3000 sunfire IV pre/pro/tuner.

I do own a Shindo Monbrison - yes. Yes, I know it is possible to make a really excellent one-box line/phono unit, and in a small package at that!

It is, as I said, 'theoretical'. There are too many variables to make apples-to-apples comparisons. (I suppose you could do that if you took the same line/phono design and implemented it both as two-box/two-power-supply and one-box designs.)

the internal phono stage on my entry level Mac 6300 integrated is as good as my Graham Slee Era Gold external.