I basically have a similar setup...I hold the MP3 and the miniamp. And can use it just like that.
If I want louder sounds or the sounds "away" from me....I have the power horn but also have 100 feet of cable.
I used to carry it in a backpack and it only took a minute to run the horn out if I needed it.
I dont know why mine failed, except maybe the batteries would go dormant from the cold (it worked great during the fall) and as soon as I would take it back in to the warm, it would start preforming better.
Although I found out Foxpro's dont last real long in subzero temps either.

I do like how you have that little setup...I was considering trying to use some mic transmitters to make it a remote unit and do away with the cord.
I have seen them made out of those enormous thermal mugs also