I don't think a schematic of the RIAA preamp circuit in the Kenwood receiver was ever posted, so who knows what the answer is to the OP's question.
The Audio Technica cartridges mentioned are moving magnet types, so will be affected by any capacitance load presented by tonearm cable and preamplifier input stage. The optimal loading will be dictated by the output impedance of the MM coil and its inductance, with the added capacitance from tonearm and interconnect cabling. The AT carts are made in such a way that they should not be loaded with any more than 250pF total parallel capacitance. Keep in mind that the tonearm wiring may have 30pF capacitance, and the interconnect cable 100pF to 150pF (sometimes more). That leaves only about 100pF maximum input capacitance allowed from the RIAA preamp. An FET input discrete RIAA preamp may have higher input capacitance than this. A 12AX7 based tube RIAA preamp is practically guaranteed to have input capacitance of >150pF, which will cause a resonant peak centered around 8kHz in the cartridge's electrical output. This can be perceived as overly aggressive sounding upper-mid and/or treble response and/or exaggerated surface noise. Opamp based RIAA preamps will have extremely low input capacitance, which is a good thing in the case of these AT MM cartridges.
However, some opamp-based RIAA preamp designs have additional parallel capacitance added for 'tuning'. If this additional capacitive 'loading' can't be defeated, then that could be a downside for these AT MM cartridges that work best into a low capacitance load of 100pF to 200pF total.
