Personally, I prefer separates (preamplifier and power amplifier) to integrated amplifiers for several reasons. First and foremost, the power supply requirements are so very different, opposite really, for pre amps and power amps.
It makes perfect sense when you think about it. Preamplifiers are all about passing along delicate signals, not powering loudspeakers. Phono stages, and especially moving coil phono stages, are handling extremely low and delicate signals. My Denon 103R has an output voltage of just .32/.31 mV! (see illustration below – this is a scan of the hand plotted frequency response that came with the cartridge, thank you Denon!). If you’re intrigued by this, check out (The formidable Denon 103 vs. 103R low output moving coil phono cartridges – is there really a difference?…).
Such low signal levels can be easily and drastically effected by the strong EMI (electromagnetic induction) of even the very best power transformer. Imagine the 20 or 30 pound transformer found is some of the best Class A power amplifiers in the same chassis as this delicate signal… bad idea. Especially with the large, high current amplifiers necessary to drive the difficult impedance loads of planar loudspeakers.
Many of the high end preamplifiers go as far as to separate the power supply into a separate chassis with a long cord so you can place it as far away as possible from the signal circuits. Nelson Pass was one of the first to implement this methodology with his Threshold preamps from the 1980s. He went even further by putting the phono sages in a separate chassis with yet another separate chassis for its power supply. Overkill?… In most instances, yes.
But it very dramatically illustrates the point of why having the pre and power amplification stages share the same power supply and transformer is such a huge compromise. That’s why you simply don’t see it happen in high end audio gear.