Equivalent of SQL Not Exists in MDX (or finding another way to accomplish same result)

I am trying to help a contractor implement a cube in Multi-Dimensional SSAS 2017 and do not have very much experience in MDX. I have a table that looks something like this: The goal is to only include rows in the query that have the lowest [seq] per each combo of [Part], [CustomerID], and PersonID after filters have been applied, and then sum the value column. So if no filters are applied, only rows 1 and 3 should be returned and the sum of the value column should be 2. But if the user filters data for only the month of June, only rows 2 and 3 should be returned and the sum of the value column should be 1. We accomplished this in SQL like this: SELECT SUM(Value) FROM (SELECT * ,ROW_NUMBER() OVER (PARTITION BY CustomerID,PersonID, Part ORDER BY SEQ asc) AS Seq FROM Table WHERE DATEKeY BETWEEN @StartDate AND @EndDate )A WHERE Seq=1; But the contractor is having big problems with performance of the MDX rank function. Not having very much experience with MDX, I am having a hard time knowing what alternatives there are that will perform well and the contractor has not provided any alternatives that match our business need. Another approach I came up with is this: SELECT SUM(Value) FROM [Table] AS A WHERE DATEKey BETWEEN @StartDate AND @ENDDate AND CustomerID=@CustomerID AND NOT EXISTS(SELECT 1 FROM Table AS B WHERE B.Part=A.Part AND B.CustomerID=@CustomerID AND B.PersonID=A.PersonID AND B.SEQ