The sack truck will be too bulky for the car. I'd like to keep the amp fixed to the device when loading/unloading. Also I do not think that the Ruck Sack thing is gonna harm your spine in any way. Those guys have been designing alpine equipment for ages.
Carrying an amp on one side and walking not upright, that is what actually will harm your back, not an evenly spread load that you carry in totally straight position. I think I'll go for it and see how it works out.