The point is: My plumbing is not "flawed," it was done per code when
it was built. This suds problem occurred recently. While I thought
the problem was a drain or vent blockage - it turns out that we
recently switched to "Tide with fabric softner" about the time this
problem started happening.

So I have two choices, rip up everthing including the concrete floor
and replumb the drain waste lines -or- change detergents and have no
problems.

A no brainer if you ask me.

The purpose of my original post was to inform people that they should
try different soaps when trying to diagnose this problem. This is a
problem that comes up freuqntly on this board - and the answer is
always "your vent is clogged, a bird or animal probably died in it."