I have 2 3.5 inch Polk Audio DB 351's up front with the adapter found in the new products section and a pair of 6x9 Polk DB 651's in rear. Great sound from these with the Retro Sound unit I have. My rear deck has a 6x9 and a smaller 5x8 opening, I used a spacer from an old 6x9 to lower the one side and it works perfectly.