After some weeks of try and error with the mod at the back of the driver I am quite sure I found my sweet spot. In europe I could not find this green felt solderdude was using in his mod. So I tried and tested a lot of compareable felts with not convincing results. The hole reacts VERY sensitive.. minimal density differences of the felt causes too much loose of bass or simply close to non effect at all. A peace of chair felt pad protector at the back of the driver with a punched out hole with 4,5 mm inside diameter seems perfect. Sublime side effect is a dampening of the driver ... kind of lawton mod lol. A pity I have no measuring station now to see the FQ The pic is with a 3,5 mm hole version which makes the 58X to a pretty neutral one. A version with a 4,5 mm middle hole delivers still nice subbass just without any whooliness now ....wonderful.