I had to replace a single tire on my 2015 GTI (side wall damage). After a couple months, I notices a humming sound that increases as speed increases. At first I thought it was a wheel bearing, but the local VW dealer said that because I purchased a single replacement tire and did not have a four wheel alignment, the two tires where "cupped". I have to replace the 2 tires and wondered if I should replace all 4 tires? Also, any recommendation on a tire? I do not race, but drive it pretty aggressively around town. Thanks in advance for all comments.