who feels that a single-chassis line/phono stage is a compromise? Do the advantages of a dedicated power supply mechanical and electromagnetic separation outweigh the disadvantage of another pair of interconnects?
Over the past couple years I've covered four standalone phono stages, two linestages, and one full function preamp with comparisons in different combinations. From my experience, it depends on the equipment - how, and how well, the linestage, phono section and power supply circuits are executed, parts quality, chassis, cables, etc, etc. There are too many variables to establish a general principle that is true the majority of the time.
As with most gears, there are synergistic combinations, there are always trade-offs, and there are different personal preferences.
That said, the full function preamp designer has a bit of an advantage. He know's what linestage will be used. Granted, makers of separates (ARC for instance) will no doubt achieve a nice level of synergy when their own gear is paired together.
Remember that over 50% of the gain in an analog rig will come from the phono section. If signal is lost there, it ain't coming back downstream.
The ideal result might be described as coming from an unfettered continuum of gain stages, the contributions of each you cannot detect. Posed as your general theoretical question: What is most likely to yield that?
Tim