If you enable bigd debug (b db bigd.debug enable / disable) and check the log file (/var/log/bigdlog), do you see a valid response being marked as failed? What does the debug log show for the response which should contain "Hello"?
RFC2616 states the delimiter between HTTP header lines is a \r\n not just \n. Depending on version, bigd automatically appends one or two \r\n's to the send string before sending it to the pool members. Could you try changing the send string to:
GET /HealthCheck.aspx HTTP/1.1\r\nHOST:www.xyz.com\r\nConnection: Close\r\n
or
GET /HealthCheck.aspx HTTP/1.1\r\nHOST:www.xyz.com\r\nConnection: Close
Aaron