I can see one is mode superposition solution and the other is FULL solution. But you also mention "The pre-stressed/initial conditions for these 2 runs were different.". Is there a static pre-stress condition in one case? I assume not since both have peak at same frequency.
Are you making sure that same level of damping is applied in both simuations? The FULL method seems to have a sharper peak than the MSUP results. Also in the modal analysis how many modes are extracted and what is the highest frequency extracted. You want to extract modes well beyond 27 Hz.
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