Yes seating makes a huge difference, at least for me it did. I've been using the same couches for a year but recently bought some cheap HT Seats from Amazon.
The TR change a lot, unfortunately for the worse. I have an 18" sub a few inches behind our seats so it's obvious TR, but it decreased when changing seating.
For my situation, I believe it decreased because the new seats are kinda hollow and much smaller so there's less surface being hit. This is just my guess explanation for why I lost TR. It's so much that I almost sent the seats back and kept my couches but I love the look of the room and had to sacrifice some TR for a nice view.
Hope this helps.