The problem is there's a range of what can be considered a "neutral headphone," and depending on one's frame of reference, something on the fringes of neutral for one person could be easily construed as not by another. For me, it's a lot easier and more beneficial to say that the Signature Pro is, by Ultrasone's standards (based on their other headphones), one of their most neutral sounding headphones.
People like to summarize the output of a particular headphone manufacturer as being a "house sound" often times, and while I think it validly applies in some cases (Audez'e, Denon), I think it's an over simplification in others (Audio-Technica, Sony). With Ultrasone, the stereotype tends to be a massive bottom-end, detailed mids, often sharp, painful highs. The Edition 8 fits this stereotype better than the Signature PRO, but both are more refined overall compared to the PRO models.
In other words, the Signature PRO is not so stereotypically 'Ultrasone' to my mind, especially compared to their recent directions with the Edition 8 and Edition 10. The Edition 9 to my mind epitomizes the pinnacle of the 'old' Ultrasone, their other sound that is smoother and not as strident. The Signature PRO then occupies a mid point of sorts between the Edition 9 and Edition 8, their old ways and new ways.