Before you swap tubes, you might swap the output channels from your preamp to see if the 6dB difference follows the swap.
If the 6dB follows the preamp IC swap, then the problem is not with the amp, but with the preamp or your source.
Then, swap the output ICs from your source. If the 6dB difference follows, then it's your source. If not, then it's the preamp.
If the 6dB follows the preamp IC swap, then the problem is not with the amp, but with the preamp or your source.
Then, swap the output ICs from your source. If the 6dB difference follows, then it's your source. If not, then it's the preamp.

