Hi, I ride both a Nomad and an Audax classic, and need to be careful with the lower back.
As far as transmition of vibration and shock through the seatpost, the Audax has more kick, due to shorter chainstays and narrower tyres.
I use a suspension seatpost, a USE SX, which removes the worst of it and a gel saddle still more. If you want to float on air, then a parallelogram seatpost can do that.