You have to consider what the impedance match requires. The amp is no doubt looking for a 4Ω load, check the back panel. What is the Barefaced cabinet's impedance. If it's 8Ω the amp will not be safe running at volume into the single cab.
It could be almost anything but careful diagnosis will reveal the problem. I do caution you that a tube amp contains LETHAL voltages even when switched off and unplugged.
You can check if you have the polarities correct by applying a 9V battery across the speaker cable. Both drivers should move in the same direction at the same time.
If the amp works as it should with your bass plugged into the Return jack try connecting the Send and Return with a signal cable and the bass plugged in as normal. Sometimes the normalizing contacts on these jacks can get corroded and fail to make proper contact.
The 6L6WGB is from an older less able family than the 6L6GC currently used. I may even have a couple of these tubes in my tube caddie. LOL
Edit to add: If memory serves me correctly the W signifies a tube designed for industrial purposes.
One pairing that you can take to the bank is never mix a 4x10 with a 1x15. It always puts the single 15 in danger as it fails to keep up with the more capable 4x10.