I recommend not less than 10K ohms, though it will work into much lower loads. At 10K, the response is down 1dB at about 15Hz, and below 10K the maximum voltage output is limited by current capability. We specify 10v maximum output, but it will actually produce over 20 volts before clipping, so that current limit is not significant in practice, where peak outputs are usually not much more than a volt.
