Write SQL query to solve given problem: In the Northeast region, what is the average household income for each city located in the state with the highest number of stores?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T2.`Household Income`) FROM Regions AS T1 INNER JOIN `Store Locations` AS T2 ON T2.StateCode = T1.StateCode WHERE T1.Region = 'Northeast' GROUP BY T2.State ORDER BY COUNT(T2.StoreID) DESC LIMIT 1