The original tube in the amp was a Groove Tubes 12AX7 (This may not be Hartke's OEM tube as the bass head was purchased used), the first replacement was a Groove Tubes 12AX7C (Chinese Manufacture) that made the amp go berserk. So far the outcome is pointing to either a bad tube or a bad series of tubes; shame because Groove Tubes touts their QC process as being extremely stringent.

If the new tube holds up for a few weeks, I'll order another as a spare.

Thanks for all the advice

John