I need to think about upgrading my own desktop speakers over the summer. What should I be looking for with passive speakers used at the PC, i.e. when the listening position is only a few feet from the speakers? With active speakers for example, you can get nearfield monitors. Is there a similar speaker type in the passive world?
Yes there are passive nearfield studio monitors (speakers), but your better just getting active monitors. The better active monitors will have trim control that will help setup in the room.
Take a studio monitor such as the Adam Audio T series, you have both high and low frequency dB adjustment, electronic bi-amping, then the engineers have matched the amplifier to the speaker.
Top recording / mastering studios do sometimes favour passive monitors, but they have far more knowledge in setup than the typical consumer, plus their amplifiers and passive monitors are sometimes in the many thousands of pounds. Consumers in home setups are normally better with active monitors from a good manufacture where the individual components (making up the entire active speakers) have been chosen to give best total performance for the budget that's available given the items final selling price.
Going back to the adjustments on an active monitor. Good recording rooms these are less important as the room is acoustically treated, plus they will have more freedom to physically move the speakers. When your room is not acoustically treated, worse your limited on your speaker placement, the adjustments on an active speaker (such as the Adam Audio above), they become more important for setup and synergy of the audio.