THE PROBLEM: I need to mount my speakers using this mount.

https://www.amazon.com/dp/B005SEN20S/ref=cm_sw_r_apa_i_YCTS5A2H1B0G1AM9WRE3_1?_encoding=UTF8&psc=1

This is the only mount that will work, just trust me. I have KLH Albany 2s, and I don't know how deep the backplate is, so I don't know how long of screws to get. I called KLH and they were entirely useless on this and wouldn't help at all. Do you guys know how deep of screws I should get, or any idea on how to figure it out