i have found that the only way i can get seamless integration with my sub (for 2 chan) is to center it between the l/r. this is most noticeable in situations like you describe, drums that move across the soundstage. this is also thiels recommendation for placement if it is primarily 2 chan listening, 1 sub.
i rarely use it for 2 chan as i think the bass is adequate without.
i rarely use it for 2 chan as i think the bass is adequate without.