Over the past few days I've confirmed that going forward users should always disconnect interconnect cables (input(s) & output) prior to initializing or making subseqent changes to the impedance level associated of the impedance settings (1 through 10). Doing so will ensure reliable results later on when you're switching between impedance levels while listening to music. By not following this procedure you invite the possibiity of there being volume level differences between each impedance setting and/or channel imbalances that shouldn't be there.
I believe the problem stems from having multiple ground connections. When nothing is connected to the preamp, the calibration process has a fixed ground reference that doesn't change regardless of impedance level. However, when multiple interconnects are attached to the preamp there are now multiple ground connection points which may prove to be either soft, noisy, or otherwise susceptible to moving about slightly as impedance levels are changed and clibration is run. As a result of these multiple ground connections, the results of the calibration process may not be consistent.
To be clear, you may not run into this problem with your preamp and system. But you can and some people have. I've been able to reproduce the problem here in our shop.
Again, the simple fix is to detach your interconnects while setting up/changing your impedance levels and running calibration for each level. Once this is done, reconnect your interconnects and run the preamp normally including being able to hot switch between preamp settings to optimize your impedance setting for your rig.
Adjustable impedance is unique tool available to Tortuga Audio preamp owners. While it can be useful to optimize performance the setup process can be a bit complex and tedious. And for most users there's no compelling need to even mess with adjustable impedance unless you're curious. But if you do, please be mindful of the above and proceed deliberately.