Defunct Company Gear Value

Yes, stuff that cannot be serviced by the original company will drop dramatically in value in the short run. In the long run, there will be a small number of these companies whose products will be deemed "classic" and the value may eventually rise on those lines. I bet a lot of MacIntosh gear will hold its value, as evidenced by the high value of many of their older, discontinued models. Conrad Johnson and Audio Research gear will also hold some value because tube gear is extremely durable--one can always service tube gear and keep it running (solid state stuff is MUCH harder to service because old transistors, ICs, etc., become difficult to find).

The wildest example of old gear holding value is Western Electric gear. Virtually anything made by that company is currently priced in the stratosphere. The amazing thing about Western Electric products is that a lot of their electronics and speakers are really VERY good, not just in comparison with other products from their times, but, in comparison with any modern gear.

Dunlavy went out of business almost ten years ago and their used speakers seem to have held most of their value considering the age.

For the most part Larryi is correct, the prices will initally drop dramatically. As J.D. points out with Dunlavy though, some will hold their value remarkably well.
Threshold is another example of a company that went out of business that's products hold remarkable value, even the Forte line.

Threshold and Dunlavy are not the norm though.