1. The cut in should be as low as possible
2. The "P" trap should be at the washing machine, NOT at the connection to the drain line
3. The trap should have a vent, and an AAV is NOT a good alternate when it is in the basement
4. IF the septic tank backs up, ALL the sewage will overflow out of that washer drain. It might be particularly catastrophic if the backup is due to a flooded drain field and your washer drain is trying to drain the whole acre.