The guys at Budda told me years ago you can normally always go smaller to larger, but not larger into smaller. So 4 ohms out can be funneled into 8 ohms (either speaker or attenuater), 4 --> 16, and 8 --> 16 successfully. But you can't/shouldn't put 16 into 8, 16 --> 4, or 8 --> 4. I just follow this principle. Someone else can chime in and explain why the transformers dont' like to see a load less than they are putting out.