Don't think you will be able to do what you ask in a single model. you might have to do this as a two pass process (import first file, export summarised version. import summarised to second model and filter/export final data) to get the information you need.
I presume you have your data in a table as follows:
Date Fight Seats Class
01/10/10 A 10 X
01/10/10 A 25 Z
01/10/10 B 30 X
01/10/10 B 10 Z
Create a summary for your data as follows:
Date, Fight and Class as Key Fields, and Seats as your Measure
Double click on Class to bring up its properties and go to the Display Tab. Select Display Values Across.
This will give you your data in the following format:
DATE FIGHT X Z SUM(SEATS)
2010-10-01 A 10 25 35
2010-10-01 B 30 10 40
Summary 40 35 75
Export that summary to a space delimited (or CSV as is your preference) file.
Save and close your existing model
Open the newly created summary.txt (or CSV) file in Monarch and create the template to extract the data into a table. You should then be easily able to add a filter "X>=Z".