The Gravitas Quartet is like a love match in a world of arranged marriages. Let me explain. Horvitz says that he was looking to join his interest in through-composed chamber music with small group improvisation in this particular format and with these particular individuals. The musical results of such projects can sound ill-matched under the harsh lights of the stage, but Horvitz’s gang seems to have cleared such cross-genre hurdles. Nothing forced, nothing awkward. Just that old cliché of wanting the best of both worlds and, in this case anyway, getting it.