There are already good advices shown. But there is one more that caught my attention: "with the MR56 set to 80 MHz" You explicitly mention that the MR56 is configured with 80 MHz, but what is it for the MR44s? If they are using 40 MHz, this could partly explain the problem. Clients tend to stick to APs with more potentially throughput also if that AP is not optimal. If not already done, use the same Bandwidth on all APs. And without knowing your environment, I would suggest to use only 40 MHz unless you know that 80 MHz doesn't cause any problems.
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