I am currently using version 1.2.25 of the LSP plugin—specifically the A/B Tester—to compare two mix versions. I’ve noticed that the plugin boosts the master level of the left channel by a few dB; the mixes themselves don’t exhibit this level shift. I’m running Ardour 9.7 on AVLinux with a Focusrite 18i20 (4th Gen). The plugin is operating on a 4-channel bus fed by two stereo channels. Could I have made a naive mistake?
I was bitten by this in the past. I found the correct routing from the plugin to be a little tricky. Here’s a screenshot showing the inputs and outputs of the 4-channel bus (fed by two stereo tracks) and the plugin as well:
Notice that only the 1st and 3rd channels of the bus are connected to the master. If you wire it incorrectly, you may end up with the output doubled on one or more channels, which in turn will result in uneven gain.
I’m not sure if this is the correct solution, but it works for me (Ardour 9.7 and LSP 1.2.29).
Oh man… I never would have thought of that—thanks a lot.
Now that the L/R balance is right, I realized the stereo image was flipped. Why does it have to be so needlessly complicated and overthought!?
But where can one find out more about using the LSP plugins?
Damn. You’re right, this is still wrong. Try this configuration:
Under the plugin’s Pin Configuration, click the Manual Config button, reduce the Audio Out to two channels, and then connect the outputs to the master bus in the standard way.
The stereo image and levels should look fine now. But who knows what else might show up. 
That was the story.
Since Ardour 6.2 Ardour comes with an A/B switch plugin (called “a-b Switch”) that does not require manually doing pin-connections, also it seamlessly works on mono (2in, 1out) and stereo (4 in, 2 out) tracks… just sayin’.
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That’s not bad at all—I actually like it better than the LSP plugin. To hear whether and how it works, I compared a completely different song first. The track I actually want to compare is identical in every respect except for one minor detail, and testing with a different song makes it easier to hear if I’ve done everything correctly.
Thanks for the tip!
So I take it the A-B switch originated from the analog console era, right? Sort of like routing both signals to a single channel on the console and playing around with the solo button. 
Yeah, these days a better option are e.g. mixer snapshots.
In Ardour you can store/recall entire Mixer scenes. You can also just do momentary recall (middle-click) and limit it to selected tracks (but not individual plugins).