Below are Geomaps of the absolute counts of phone listings for the variants of Haselbacher and Haselbach in Austria taken from the Geogen website. Recall that there were no Hasselbachers or Hasselbachs in the Austrian phone book online.
Haselbachers are most numerous in Neiderösterreich which contains Vienna and Gresten. There are 41 phone listings in Neunkirchen alone. There is also a population more to the south in the Stiermark and Berganland. Gresten is just on the western edge of this distribution. It may not be a coincidence that a search of Google Map returns a town Haselbach in the Stiermark!
The handful of Haselbachs are more scattered, including a few in Vienna but nowhere else in Neiderösterreich.
Haselbacher n= 146
Haselbach n= 9
Discussion
It is clear that there are still many Haselbachers living in Austria. Furthermore they are mostly in the same Austrian state of Neiderösterreich as Gresten. We know that at least one Hasselbacher, Adam, lived in Gresten in the early 1700s and married twice. My inspection of online Austrian church books reveals a scattering of Haselbachers living in the late 1600s and 1700s. It is quite likely that some of these modern Haselbachers are cousins. Whether we are ever able to make the connection is another matter. We need some Austrian partners to help us!
The transcripts by Schöberl (a very old Gresten family) of the oldest Gresten Church books suggest to me that in the 1600s, Hasselbacher was usually spelled with a single s. However it seems occasionally to have been spelled with two. I have available a microfilm of the early 1600s books of Gresten, but it is of poor quality and difficult to navigate. Recently, excellent scans of the book were available on line, but currently they seem off-line for technical reasons. It may be that we will be able to look at the earliest written forms of our name and draw our own conclusions.