IMHO if bass is a particular concern, I can't see the thumb screw ones providing much of a rigid platform to hold the speaker firmly in place to get the most out of the speakers you choose. I measured my brackets, and the clamps will go to 11.5". They could be used for 12" as is, but you'd have to put a small screw in the bottom of the speaker. You'd need to get new screws to expand the clamps further than 12", if you needed wider, and you'd have to put one small screw into the bottom of your speaker.Past 11.5" the clamps would hold just fine left/ right but could possibly rotate .. there are small holes in the bottom of the speaker platform for screws. Also note, that my brackets will only orient forward / down about 45 degrees... see the diagram.
The only alternative I see, is to use L brackets top and bottom, but you'd have to put screws into the top and bottom of the speaker....