Old & New "Studio" equipment

Always the deal slut, I've been noticing on ebay, etc, equipment like (old) JBL Studio monitors & amps like Hafler 9000 stuff - stuff originally sold to studio pros.
What do "they" do, that "we (audiophiles)" dont do, (and visa versa)?
How does "studio" equipment sound different?

(posting similar but speaker specific question on speaker area)