One of the poorly understood issues with preamps is that they must control the interconnect cable. The technology to do that, to obviate the effects of the interconnect cable, was developed nearly 60 years ago, but it rarely gets used in the home (that technology BTW is known as the low-impedance balanced-line system).
Outboard phono sections face this issue, just as line sections do: in most high end audio systems, the interconnect cable behaves like a filter, filtering not only frequencies, but impact and staging information as well.
The only ways to avoid this are 1) equip the stand-alone phono section with proper cable-driving ability (none that I know of have this BTW) or 2) build the phono section inside the same box with the line stage, thus avoiding the connectivity issue altogether.
Solution 2) above means that for best operation, the preamp's power supply rails had better be quiet! so as to avoid interaction.