Multiple Table Lookup From Multiple Criteria

I think any way you approach this task there's going to be some legwork done to make the data whole for lookup purposes. I used INDEX with several named items:

=LET(
    a, XMATCH(Series & " Class " & Class, tables),
    i, XMATCH(Row, SeriesAClass150[COL.ROW]),
    j, XMATCH(Column, SeriesAClass150[#Headers]),
    return, IFNA(
        INDEX(
            (
                SeriesAClass150, SeriesAClass300, SeriesAClass400, SeriesBClass150,
                SeriesBClass300,
                SeriesBClass400
            ),
            i,
            j,
            a
        ),
        ""
    ),
    return
)