I seriously disagree with those who suggest one is best off with separate phono and linestages. (And by the way, the term "preamplifier" historically assumed a linestage and phono stage on one chassis.) Here are the advantages of an all in one unit: (1) The signal runs through one fewer pair of inputs and outputs and one fewer (often very expensive and always SQ affeccting) IC is required. This point alone is nothing to toss aside as not only do ICs add flavoring, at best, but also the connectors are a source of subtle alterations in SQ. (2) Just because the phono and linestages are on one chassis does not necessarily mandate a single power supply. The best all in one units have a massive power supply on a separate chassis and often that box incorporates separate supplies for phono and linestage, respectively, and (3) having the phono and linestage on one chassis also gets rid of an output coupling stage entirely or at least permits the use of a much lower value capacitor in a capacitor coupling between phono and linestage. Also, in a tube unit, no need for a cathode-follower at the output or and a solid state unit, no need for the analogous solid state topology. As for separates, one advantage (to the manufacturers) is cost. If you really want to spend money and pay for the same thing twice (chassis's and PSs), go for it.
I own both an Atma-sphere MP1 (albeit a fairly ancient one with some updates) and an Essential 3160. Both of these units are fully balanced, inputs to output, and both have outboard PSs with separate supplies for the phono and linestages. I can only imagine the sound of an up to date MP1, but it is somewhat restricted with only one pair of balanced phono inputs, designed for high gain but can be reconfigured for low gain. For flexibility and amazing transparency, I would highly recommend the 3160 or the latest greatest iteration, the 3180, which I have not heard, in that both of these have entirely separate circuits for MM vs MC cartridges. My 3160 has an exceptionally lucid linestage which was a hard sell to me, because I am at heart a tube guy. I can only imagine how great the 3180 is going to be.