At AXPONA back in April, Rel demonstrated a line array of 510s with a pair of Wilson Sabrinas (I think... I was paying attention to the Rels). Rel not only demo'd the effect of the full line array on/off, but also demonstrated the effect of running just one 510 per side, two 510s per side, and the full stack of three per side, sequentially.
I was very impressed - the full line array added significantly to the size of the soundstage and improved the imaging in ways that I would never have anticipated. The effect was, of course, most significant with the full array but was present with just two of the three Rels engaged on each side. Of all the experiences I had at AXPONA this year, the Rel demo was the one that stayed with me... I've been thinking about it ever since the show.
Consequently, I've begun to accumulate Rel Carbon Specials to add into my system. I have 4 of the 6 on-hand (in boxes, not yet set up), so can't provide impressions yet but am certainly looking forward to integrating the six-pack.