Looking back at CJ...they did have a 3-box preamp years ago. The later ARTs were all 2 box. Their latest GAT is 1 box (and crammed to the rafters inside). But by universal accounts from everyone i could speak with who owned and/or reviewed both ART 3 and GAT...the GAT is superior and (subjectively) quieter. And in my system with 95db speakers, when i turn it on, if i have new tubes, i have to stand next to the speaker with my ear turned to the tweeter to hear it. No hum. A whisper of hiss at 3 inches in a silent room...i can stand next to the speaker at volume 1 and still hear the music louder than the hiss.
Clearly, great stuff can be done in 1, 2 and 3 boxes. In the end, the question is if the same designer went 'all out' on both a 2box and 1box, without regard to cost...purely sound quality, etc....which would be better? Many manufacturers seem to lean towards separate PSU. Esoteric (digital), Zanden, Audio Note, Pass, Krell Pre, Audio Research Ref Anniversary, Robert Koda, Wavac, Vitus, Gryphon, etc... I have personally heard the benefits of shielding from RFI, EMI...so is it better in separate boxes? seems logical.