Maybe check flatness of the bearing edges first.

No, the hoops are heat treated before finished. plenty strong.

It`s what SONOR refers to as a modern mid level kit. I think the hoops don`t have enough true contact around the head like was suggested. It`s why 8 lugs is better than six.But they should have stuck with 9.

Don't know about how Sonor does bearing edges Gilnar, but I have a couple MIJ 6 luggers that I had to redo bearing edges with my router table because of this same problem. I did go with a 'round-over-45' instead of the 'newer' -double 45 degree edge and they sound great to me. I'd examine bearing edge and trueness of hoop... good luck!

I say try a different drum

I would try self muffled heads on both the top and bottom.

I use a Coated Remo Powerstroke 3 on both the top and bottom of my floor toms for rock/pop/contemporary stuff (ie not jazz). It's a single ply head, like an Ambassador, but with an internal muffling ring. Remember that the internal muffling ring has two functions: 1) (Secondary). It SLIGHTLY muffles the ring and 2) (Primary) It lowers the fundamental pitch. By using the self-muffled heads on both sides, you are going to get a lower pitch than using a Fiberskyn or Ambassador. You may want to try that. This combination works great for me. As a matter of physics, the higher overtones are closer to the outer edge of the drum. The more you move closer towards the center of a drum, the more pronounced the lower overtones are, and when you move closer to the edge of a drum, the more the higher end over tones are dominant. I still get plenty of sustain using these heads.

Another trick to try to get a little more sustain is to take a small piece of foam and put it under each floor tom leg. That will increase the resonance. Try this simple test: Strike the floor tom with a normal, average hit while the legs are touching the ground like normal. Than lift it off the ground with one hand and strike it again. If it sustains longer when it is off the ground, the foam will help get that extra sustain, because the drum will actually be resting on a cushion on foam than on the ground directly. I do this on all my floor toms as well. Makes a big difference! They make "air suspension" type tips for floor tom legs, and those work well, but foam is easier and cheaper. You can use your existing legs/tips and just put the small pieces of foam in your stick bag.