I do a lot of work for B2B companies that sell really complex stuff. I interview their customers constantly.
And there is no question in my mind now that THE most important sales tool is something that isn't even on the radar of most marketers: the product manual.
I've been telling clients for years that there is no such thing as a "virgin environment." Someone buying something that is going into his existing environment knows that he has to make sure that it will work with all the other things in that environment.
Greenhorn buyers attempt to find that information on the website or even in PDF datasheets. They try asking salespeople. But the cognoscenti (the majority of your buyers) have been disappointed so often, using these methods, that they don't bother. Instead, they go straight for the manual, which they hope to find on the website in downloadable form.