It will definitely depend on what you're looking to add; if it's just more bass than you have now, the 8D will do it--their low end compared to the 6Ts are pretty similar. If it's deeper extension you're after, I'd really recommend at least the 10D or the 12D for noticeably better low end response and bass that you can feel as well as hear. We've certainly had folks who add the 8D to 6Ts, we've tried it here ourselves, and it sounds good; my own personal feeling though is you can do almost the same thing by boosting your bass control/eq in your receiver.

- Oliver