As long as the input impedance is resistive, and the preamp output capacitor is large enough to deliver deep bass, and the preamp can produce enough current to adequately drive the power amp, then there is no limit to the impedance it can drive.
Those are big qualifications though:
If the load device (power amp in this case) has a transformer at the input, then the impedance is not resistive and the question becomes more complicated.
The stock Quickie output capacitor is 2.2uF, which can drive 10K ohms with a -3dB frequency of 7Hz. That's quite satisfactory in most cases.
At the low-battery point (24 volts from the 36-v array) Quickie can - in theory - make around 2.0 volts into a high impedance, and 1.0 volt into a 4K impedance, without either voltage or current clipping. We have not done these measurements, this is just simplified theory. Below 4K both gain and maximum output voltage fall rapidly. And below around 50K there will be an increase in distortion and a change in the distortion character. Whether this is audible, or whether this is unpleasant, is too subjective to predict. A highly sensitive amp and/or speaker will reduce the signal level demanded from the preamp, greatly reducing the distortion in practice. As you can see, this is the most complex and situation-specific of the three criteria.
Bottom line, for those who are still with me, is ... if it sounds good, it IS good. Everything else is just explanation and rationalization.
