This is is not the exact set up as you have but only an example of a slight modification, It is Manufactured by Everlast like yours and is the same drum wood material. This unit flexed and vibrated and altered the rebound now with just the added brace(2x4's)against my ceiling eves it is solid and rebounds well. I am also new to the speedbag and lets face it starting out our equipment is entry level and once we improve our skills, our equipment can be upgraded. My oberservations are like the others have stated make sure bolts are tight and if that don;t solve it take care of it by adding some more bracing.
**Do an Experiment before you change the bag hold the drum with your hand at rebound point pulling down with tension now strike the bag with the other hand if bag rebounds well add some bracing***. Just a suggestion