With the driver stage tuning peaked and the rig transmitting on 160m with the drive control turned all the way up, what does the grid current indicate?
If there's a lot of drive and very little output, then the problem isn't in the driver stage or anything before the 6146s. The problem would be after the 6146 plates, somewhere in the output network, plate RF choke, plate coupling/blocking capacitor, etc.
If there's very little drive even with everything cranked all the way up, then the problem is likely to be before the 6146s, in the driver stage or possibly even before that.
At least this simple test helps isolate the problem to a part of the chassis.
WB2WIK/6