Is it a set up problem?

[Mike Mandelas]

Dear friends a new double bass has arrived in our possesion. It is a Hungarian instrument, built in the 70's, fully carved, with beautiful flamed tonewood. The arco sound is excellent (strung with Evahs). The piz sound however is not so good. I strung it with Spiros Mittel and it improved a bit but it remains inferior compared with the arco sound. I experimented with the soundpost and the bridge but to no avail. Any ideas will be wellcome.
Mike