I think the problem is that the 500 ohms is only the nominal impedance of the phono stage, and tubed phono stages can actually have output impedances several times the nominal impedance, especially at lower frequencies.
That’s right, Scott. Most tube-based phono stages and preamps (as well as some solid state designs) utilize a coupling capacitor at their output. The impedance of a capacitor increases as frequency decreases, and as a result it is not uncommon for the output impedance of such a design to rise from a few hundred ohms at mid-range frequencies to 3K or 4K at 20 Hz. (That rise can be reduced if the designer chooses a larger value capacitor rather than a smaller value capacitor, but doing so can have other downsides).
Since the 20 Hz output impedance of your Croft RIAA phono stage doesn’t seem to be indicated anywhere, and since it presumably uses a coupling capacitor at its output, I had suggested that to be safe a factor of 50x to 75x should be applied to the 500 ohm nominal output impedance, resulting in the 25K and 37.5K numbers you cited. Note that the 25K figure is not even all that conservative, as an output impedance rise to 3K at 20 Hz (which is not uncommon) would not meet the commonly stated 10x guideline, at that frequency.
What I’d suggest at this point is contacting Croft and asking if they can tell you specifically what the output impedance of the phono stage is at 20 Hz. Or, alternatively, what they would recommend as the minimum load impedance that should be used with that phono stage.