Koma-script, change chapter appearance (how to add a circle and horizontal ruler?)

Here's a solution using the titlesec package; this will work as expected with both numbered and unnumbered chapters; taking into account the comments, I've replaced the initial definition of the circle with \CIRCLE from the wasysym package:

\documentclass[chapterprefix=true]{scrbook}
\usepackage{titlesec}
\usepackage{wasysym}

\titleformat{\chapter}[block]
  {\normalfont\huge\sffamily\bfseries}
  {\CIRCLE\vskip6pt\titlerule[1pt]\vskip9pt\chaptertitlename~\thechapter.}
  {0.5em}{}
\titleformat{name=\chapter,numberless}[block]
  {\normalfont\huge\sffamily\bfseries}
  {\CIRCLE\vskip6pt\titlerule[1pt]\vskip9pt}
  {0pc}{}

\begin{document}

\tableofcontents
\chapter{Test Numbered Chapter}
\chapter*{Test Unnumbered Chapter}

\end{document}

Some images of the output showing a numbered and an unnumbered chapter: