You can easily mount these brackets on drywall and support the weight of those speakers. The B-Tech manual shows in panel 1 that each bracket has 4 holes for mounting screws aligned vertically. That will allow you to mount them directly into a wooden stud.
http://a248.e.akamai.net/pix.crutchfield.com/Manuals/121/121BT77B.PDF
The manual also shows that it comes with 8 mounting screws and 8 plastic screw anchors. Keep the screws, but throw away those useless plastic anchors.
If you don't have one, get an inexpensive stud finder($15 or less), such as
Zircon 62168 StudSensor - Amazon.com.
These stud finders allow you to easily find the edges of a 2×4, so you can always be sure you're drilling into the center.
If your studs aren't where you need and you must locate the speaker brackets on hollow drywall, you can use toggle bolts, but I prefer using molly bolts. Because your brackets have room for 4 mounting screws, I would expect that 2 normal size molly bolts per bracket would be enough. You are limited by the size of the holes in the bracket, so check that before you buy the largest heavy duty bolts you can find.
This web page explains all the different options you have:
Using Wall Anchors Including Molly and Toggle Bolts, Plastic Anchors and Expansion Anchors