If your ultimate plan is to have sub/subs, then 6x9's are pretty useless. If that's the case, then I'd get a set if 6 inches and put them in for now, as cutting the parcel shelf is illegal. If you can't take them back, then your best to get spaces (bricks) under the parcel shelfs carpet so it all looks stock. But if you don't car about the roadworthyness, then just cut the shelf. I can't talk, I did the same thing