Unfortunately the specifications on the phono section are sparse so it is unclear what the input configuration is. The fixed 100Ω value is an odd choice unless there is an internal SUT for the MC input. To further confuse things the E-250 phono has an internal SUT and lists gains of MM: 38dB, MC HIGH: 57dB, MC LOW: 66dB and input impedances of MM: 34kΩ/47kΩ/56kΩ/100kΩ(switch selection) MC HIGH: 40Ω, MC LOW: 2.5Ω. When I do the math the numbers do not add up without some overly complex switching and loading arrangement. The MC high setting it adds 28dB of gain which suggests a 1:26 step up ratio. That means that the impedance ratio is 1:676 so if the 2.5Ω value is indeed the actual value then the secondary of the SUT would need to be terminated with 1700Ω which is highly unlikely. Looking at it from the other direction and assuming the 1:26 and a 47kΩ load the input impedance would be 70Ω. If you work out the same numbers for the MC Low they are equally confusing. This leads me to believe that 2.5Ω and 40Ω numbers are not the actual input impedance but rather the suggested cartridge impedance which is how many Japanese companies specify phono inputs. The E-500 lists that it uses 4 SUT's rather than the tapped unit in the E-250.
Since the specifications of the phono section of your integrated do not list the MM and MC gain, it seems the only way to find out for sure is to measure the DCR of the input of the phono and see what you get. If it is 100Ω then that is likely your actual impedance value and it may be possible to change that to a higher value. If it is around 1Ω then it likely has a internal SUT with a step of ratio of ~1:20. My guess is it is the later at which point while increasing the load value is possible it may lead to less than desirable results.
dave