Seperate Power Supply and Preamps

Several of the top end tube preamps like Hovland, Joule Electra, Herron, and Lamm have no seperate power supply. Others like First Sound Presence, Atma-sphere, Cary, have seperated out the power supplys with some of their models. What are the advantages of this sonically or is this just a physical limitation problem with the manufacturer?
Several possible reasons. The first six that come to mind are:

Size limitations.
Isolation from proximity of high voltages.
Isolation from magnetic fields and transformers.
Splitting the heat sources into two or more chassis's.
Dividing the weight into more manageable packages.
Ability to apply different isolation techniques to each piece.
Albert provides a good list. Isolation and size limitations seem most common. If you look at the power supplies in some of the components that use a separate chassis, you'll see that they are large enough all by themselves to equal the size of many separate preamps or power amps. For preamps, an often cited rationale is to move stray electomagnetic fields generated by the power supply components as far from signal-bearing circuits as possible in order to reduce the possibility of interference.
Musical Fidelity claims that their separate power supplies allow the sound to remain clear and open as the volume level is increased. Something about stronger fields being created by the power supply as the power demand increases. Vibration from the power supply contaminating (not in this case) the line/phono stage, etc., would be an advantage as well. I have one of the MF X amps (integrated) and the power supply weighs about as much as the rest of the amp.
Everyone has elegantly stated the positives of outboard power supplies. What about the negatives? I can name two. 1)RFI riding on the cable conneting the supply to the preamp will go directly to the circuit as opposed to RFI riding on a power cord which goes into the transformer. 2)Cost.
As for the above post: Most companies should provide a RFI filter at the incoming connecting cable side or in the cable itself. Secondly as far as cost is concerned, I think isolating all the nasties that can come from pwr supplies should give piece of mind when it comes to your gear and the money invested. Just my opinion.
One of the Reasons the Rogue Preamps sound so nice.Seperate power helps keep everything in place.
Outboard power supplies are only beneficial if the wiring harness connecting the supply and the main unit are of the LOWEST resistance possible. This means that the manufacturer would have to use heavier wire than if the supply was located in closer proximity ( same chassis ) as the "motherboard". As mentioned above, RFI needs to be taken into consideration also. This is not to mention that you not only have the power cord from the power supply to worry about, but you've also got the "umbilical cord" from the supply to the main unit dangling too.

Many newer designs that DO sound good go just the opposite route. Instead of a remote or centrally located power supply, designs like McCormack places the filter caps AS CLOSE to the main output devices as is possible. This gives INSTANT power on demand as fluctuations occur with the shortest signal path possible. While i am primarily discussing power amp design here, preamps should be no different. Even though the signals that we are dealing with are much lower in level, so is the amount of magnetic field generated by the transformer in a preamp. This is due to the miniscule amount of current that a preamp pulls.

I've had preamps of both types: internal supply and external supply. Besides the outside factor of convenience, I'll stick with a well built "all in one" unit. That is, until i find something that performs better. Sean