I recall that the designer of Lyra cartridges, generally believes that, with a good phonostage, cartridges should be run pretty much wide open (minimum loading), which means at the 47k ohm setting. The undampened high frequency peak would only be a problem with gear that overloads or oscillates from being fed high level, high frequencies.
I have owned several Lyra cartridges over the years-- Lydian, Helikon, Titan (I still have this one). I have found them to be somewhat less sensitive to loading than other brands. But, the ideal setting is one that is determined by the specific phonostage being used. In one stage it might be wide open in others, the richer tonal balance from some loading is a plus. In my own system, I never wanted a loading resistor value lower than 150 ohms with any of these cartridges. More loading (lower value) meant a rolloff of the highs and a sort of lifeless sound.
Overall, if one had to make a phonostage with only two values, the choice of 47k and 100 ohms represent pretty good choices. I have never heard a setup where I thought a lower value than 100 ohms was a good choice. On the other end, there is very little difference in sound for any value from, say 1000 ohms on up to completely wide open. I once heard a system that had all sorts of problems with noise that we eventually traced to RFI interference. It turns out that the default setting for the preamp phonostage was 100k ohms. Increasing the loading to 47k ohms or more killed the RFI problem.
I am going to guess that you are the one who posted a similar question on Audio Asylum. John Elison's suggestion on homemade loading plugs is a good way to experiment. I have made similar plugs from cheap parts from RadioShack. Plug in different values and set the phonostage to 47k ohms loading. If you do the ohms law calculation for resistance in parallel, you will find that for most of the values you should be trying, the value of the loading resistor will be close to the value of the combination with the built in 47k ohm loading. After you find the ideal value, you can then order much higher quality resistors from an electronics supply house (e.g., Vishay) and either solder these into a good quality plug, or do what I do, which is open the phonostage up and solder the resistor in between the hot and neutral pin at the back of the input RCA jack (to eliminate unnecessary junctions from the experimental loading plugs).
