I bought a pair of these amp clamps in order to mic up the two Vox AC30s we use in our band. Prior to this we had simply hung our Sennheisser mics from their xlr leads slung over the top of the amps and secured in place by black gaffa tape. Sometimes however, the tape can come unstuck, so these clamps seemed to be an ideal solution. However, we're not happy with the clamps so we've gone back to the gaffa!
The AC30s are big amps, but the spring loaded clamps do manage to stretch around the cabinets and hold firmly in place. The main problems with these units is the goose neck section. It's not very long, so you may struggle to get it to stretch across to achieve a decent position in front of the speaker cone in a large cabinet. Even then, the goose neck is not of particularly high quality: it does not tend to want to stay in one place, and droops under the load of even a light weight mic.
The Sennheisser E 906 mics that we use are designed to be positioned flat in parallel with the grille cloth, and it is pretty much impossible to achieve this position with the goose neck provided. I suspect you'd have better luck positioning a mic like a Shure SM58, where the mic is positioned tangentially to the speaker - especiallly with a smaller amp or cabinet.
