My search shows this to be a vented design, and the amp does employ some type of feedback to reduce distortion. I suggest a search for an exact replacement on ebay, or contact Velodyne for repair. They may charge you a flat rate which may be cheaper than a decent replacement.The CT 150 is a sealed design and chances are the original amp had some degree of boost in the 30 hz range. Since the measured response of the sub starts to drop off at 25 hz a replacement amp is going to need a sub sonic filter in the mid 20 hz range. The Bash 300 is up to the task if you're willing to change 2 resistors. The amps default subsonic filter is 17.7hz but it can be changed to 25 hz wth 2 db of boost in the 31-35 hz range.
With the sub being 15 years old I would not recommend a 500 watt amp.