Mostly the listening area. I no longer have to sit at the MLP to get the same soundstage and imaging. The bass and lower end seems to be increased as well. I didn't realize my sub was unplugged the other day and was enjoying a listening session and I couldn't believe how much bottom end it had with the sub off. With the 100s I could tell a difference between the sub on and off. But to be fair and honest I never tried the 100s with no sub after I got the 1500 hooked up. I'm positive that would have made a noticeable difference.