03-25-12: Alonski
It seems clear and obvious (well, now) that "direct" is not possible because with any outboard or built in phono stage, one would need a volume control and additional circuitry to control the downstream amp, in essence, you need a line stage for this to work whether it's part of the Steelhead or VAC is irrelevant.
FWIW, and I have no familiarity with the Steelhead, my take on this is that it is not irrelevant. The functionality of a line stage is needed, somewhere, but if that functionality is implemented to a high standard within the same component that provides the phono stage functionality, investment of a given number of $ can potentially provide better results, since those $ are only paying for one component instead of two (plus an extra pair of interconnect cables).
Also, while a pure phono stage (lacking line stage functionality) will not incorporate a volume control mechanism, it still has to include circuitry to drive the line stage and the interconnect cable between the two components. The requirements for that circuitry will not differ greatly, if at all, from the circuitry that would be required to drive the power amp, and the corresponding interconnect cable. So the two-box approach means having an additional buffer/driver stage in the signal path, compared to the one-box approach.
Obviously, those potential advantages of the one-box approach may or may not be realized depending on the specific components that are involved, system synergy, listener preferences, etc. In your particular case, my feeling is that what you previously proposed is likely to be well worth exploring:
Replacing an amazing preamp that has a really good built-in phono stage, with an OUTSTANDING phono stage that allows me to go direct to my VAC amp for best LP sound (which is what I care about most) and would accommodate my CD player with what would probably be a very decent line stage input. So in essence, it's a preamp with its main focus on vinyl.
One more benefit to this is that I could sell my VAC pre to offset some of the cost of the new phono stage.
Best regards,
-- Al