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Panel based surge protectors have a high let-through voltage. And you also need the second surge protector to be at least 30ft downstream to allow for resistance to reduce the surge energy that reaches the 2nd surge protector.
There's a reason 6000V3000A was chosen for UL1449. It's the worst case scenario for anything other than a direct lightning strike. Of course if you get a direct hit, nothing will save you outside of a serious protection system with lots of air terminals around your roof. Even then, you're still probably fucked.
As I have to keep saying over and over, you should use both whole house and point-of-use surge protectors.
6KV/3KA is just one of many levels used in different aspects of UL1449 testing. 1449 currently supports up to 20KV/10KA, and will likely be extended for standardization. For outdoor equipment, you don't need a direct hit to get more than 6KV/3KA. 6KV/3KA is likely the worse you will see indoors in a commercial environment unless you get a nearby hit which is why it was chosen.
You are absolutely right, a layered approach to surge protection is the best thing you can do. Get the biggest whole-home unit you can justify, and then use point of load surge protection. The surge protection built into your equipment is absolutely no substitute for point of load surge suppression. w.r.t. the 30 feet, absolutely any distance is an advantage, and it is not just a matter of resistance, the substantial inductance of the AC lines also is important to reducing voltage. How important is a factor of what type of surge you are dealing with. Unfortunately lightning based surge is longer in duration so inductance is not as big of a help, but can't hurt. It absolutely softens the voltage peaks and can allow time for the whole home to clamp before a jump over voltage reaches your equipment.
With layering, your whole-home will limit to say voltage X, which could be say 2000V. The combination of the resistance and inductance of the wires in your house, and the point of load surge suppressor will drop that peak to <1,000V, or perhaps higher voltage, but with less energy. That is enough for the surge suppression in the end equipment coupled with the usual inductances, etc. to survive. Most equipment targeted for the home has some surge capability, but it is pretty limited. The transformers and capacitors in linear power supplies make pretty good surge suppressors. That is essentially how a SurgeX works. Some inductance, a diode bridge, and a capacitor floating at the peak of the line voltage to absorb the surge.
The non-sacrificial surge protection, i.e. like SurgeX, and what is in some Furman's, etc. is good for short duration surges, not as good for longer surges. Combining sacrificial with non sacrificial gives you best of both worlds. The long surges have more total energy, which will eventually pop your MOVs, but that is what fuses are for. When you have high enough energy, it is surprising how fast a fuse can blow.