Phono Stages with External Power Supplies...Worth It?

It all depends. With both preamps and phono stages, separating the PSU from the line circuitry is in theory optimal. Let me emphasize in "THEORY". This is because there are phono stages and preamps with built-in transformers and capacitors (PSU's) that are superior to others that have separate with the PSU tied to the line stage via umbilical. Separating the boxes is expensive. So the short answer is that with anything less than state of the art, you are generally better off with a one box solution but not always. Generally. My Manley Steelhead has a separate PSU but there are likely (scratch that, definitely) better phono stages that are one-box. My ARC Ref 6 preamp is one-box but there is little or nothing on this planet (IMveryHO) that beats it. But then, there is the Ref 10 that sounds different, but not necessarily better. 

fsonicsmith, I am surprised you would place the AR Ref6 above the Steelhead, but if you are a "solder slinger", like me, I can suggest some very small changes to the Steelhead that make it sound worlds better. Either PM me or indicate here if you want the information and my suggestions.

Lewm; I don't. They are two separate categories. The Steelhead is a phono stage and the ARC Red 6 is a preamp. I used them as examples only. At the outset of my post, I mentioned that preamps and phono stages are very similar and they are. They both take a low voltage signal and apply attenuation to them. They are both very susceptible to EFI/MFI from transformers. Further, I love my Steelhead and if Mike Fremer is correct that it is no longer reference quality, I don't care. I would never mod it. The folks at Manley are incredible with their product support and I would never jeopardize my warranty and their support with mods.