SWITCH statement in DAX using a "between this value and this value" as a filter?

The SWITCH statement allows comparisons with constants only.  You need to use an IF statement.  For a calculated column, this is an example:

Pop Classification =
IF(Population[Pop] >= 1000 && Population[Pop] <= 25000,
1,
IF(Population[Pop] >= 25001 && Population[Pop] <= 50000,
2,
0)
)

I copied exactly what you posted and it worked for me.  Make sure your data is of type Decimal and not Text.

Figured it out... or at least partially... it's summarizing data when it shouldn't so it's adding it somewhere.

So all of your suggestions have helped! :) Thank you!!

I was about to tell you that.  Check the table fields for an aggregate function.