Thanks for the post. 15 x 19 is quite large. Hence it is not a surprise that 3 subs work better than 1. Yes, the S510 costs considerably more than the T9x. Just unsure if there will be a noticeable difference if doing a one to one swap ie. from the PMC to REL T9x / S510.
Will decide and take the plunge when ready. The system sounds fine as is.