I have been using the first generation Echo for ages connected to my home amp, and thought I should give the Echo input a go. Either way it offers no advantages/disadvantages to an Echo other than cost.
It works the same, and the sound quality is much the same. So if you want to add bluetooth wireless streaming to your old/new amp then this is by far the best and cheapest way to do this.
I have this unit connected as a room group with an Echo Plus and Echo Sub with speakers around the pool. Everyone comments on the great sound quality achieved.