Fish,
Here is perhaps one of the least expensive amp options out there I have seen that has similar power to your integrated but also advertised ability to double down to 4 ohms. SOme good reviews out there on it, though I have not heard, but low cost might help make it a viable option to consider at some point. Do the SPL meter test first though in order to get a clearer handle on what is going in currently.
Class D audio amp
Here is perhaps one of the least expensive amp options out there I have seen that has similar power to your integrated but also advertised ability to double down to 4 ohms. SOme good reviews out there on it, though I have not heard, but low cost might help make it a viable option to consider at some point. Do the SPL meter test first though in order to get a clearer handle on what is going in currently.
Class D audio amp