Powdermonkey,
So.., ! I am not the only one who has issues with the ground thumping.

Here' my short story:
For several years I have used my listeroid which is SOFT mounted to the garage concrete floor located 25 feet from the house.
The generator base rests on 4 vibration isolators which are supposedly rated for the weight of the unit.
I suspect that while they are the correct weight rating, they are not rated for very low frequency power pulses and are better suited to higher rpm multi cylinder engines. I also worked very hard to make the exhaust almost silent so that the neighbors cannot hear the system running. All seemed good. Then last month I was running the engine to heat the greenhouse. When I went inside the house I heard a tapping and bumping sound. I went into the basement and could hear 2 distinct sounds. One was a plumbing pipe vibrating at the same frequency as the engine, and the other sound was a deep bass thump coming through the walls of the foundation. :-(
Remember, this is 25 feet away from the generator. I got concerned that my neighbors may also have this thumping in their foundation.
This is when I got the idea to move the generator to the country to charge batteries at a cabin. So I researched how Lister recommended to mount the engines back in the 60's. I was under the impression that a 1600 pound concrete block would eliminate the ground thump. The ground thumping while trying to sleep at a cabin is also not acceptable.
Your story is very useful and will potentially save a lot of time/effort.
Alternate method:
I did come across some Marine engine vibration isolators which are quite soft and would probably absorb most of the power pulse, however I suspect that the engine may also bounce around a lot. What is your experience?
There are a lot of previous threads relating to Lister mounting and I have read most of them but there doesn't seem to be any one solution that fits all applications.