Hi David,
If you want the entire speaker to be isolated from sending unwanted vibration into the shelf and other connected shelves and components, I would recommend two large size compliant isolation feet under the rear of the speaker and two smaller size compliant isolation feet under the front of the speaker.
Best Regards,
Barry Kohan
Disclaimer: I am a manufacturer of vibration control products
If you want the entire speaker to be isolated from sending unwanted vibration into the shelf and other connected shelves and components, I would recommend two large size compliant isolation feet under the rear of the speaker and two smaller size compliant isolation feet under the front of the speaker.
Best Regards,
Barry Kohan
Disclaimer: I am a manufacturer of vibration control products