Regarding thread sizing and footers in general, Dr. West provided the following information January, 2020, for my new Majestic 845PX’s:
The speakers each use 5 footers, and the mounting thread is 1/4-20. The length is 1″.
We use carpet glides as standard since it makes moving the speakers easier. There are quite a few footers available on the market and it’s best in my opinion that customers research what they feel works best for them.
Spikes have a universal appeal from an appearance
standpoint, and we use them on our Ultimate series, but they have no acoustical advantage due to the fact that the vibrantional
energy density of the speakers is relatively low and is distributed over a larger area. In contrast, high-intensity small-box speakers would benefit using spikes since it would relieve the enclosure of some of the vibrational energy.
In your situation wherein the floor is concrete with a covering of carpet, I personally would recommend the carpet glides from a functional standpoint, but then there’s an aesthetic factor that can’t be denied which might suggest a different footer design. Hope this helps.