If you have the input switching and gain control built into the amp, the need for a passive pre is pretty much academic. But, if you use a CD player and a power amp and simply need to attenuate the CD player's signal, a pot in a box is a good idea, although it tends to attenuate high frequencies more aggressively than others.
As such, it needs to be combined with care (generally, a pot in a box has lower input impedance and higher output impedance than active gain stages, so the capacitance of the cable becomes crucially important - in reality, this means interconnect cables about 20cm long at maximum).
As to the Passive Magnetic preamp... you got it, it's a pot in a box with a pair of screened transformers. No active gain stages. Not bad for nearly £3,000. And it's not alone - check out the Audio Synthesis Passion preamps (passive preamp with active remote control pot, £1,300), and there used to be a Cello passive pot, made up of a handful of bits of silver wire, an ALPS pot and a alloy case. It was nearly £800 when sold 20 years ago - that would be nearer £2,000 today.
Perhaps now you see why the OBH22 begins to look like a bargain among the built passive preamps!
