So I put together a little test circuit to test out a NPN BJT buffer circuit with a TIP122 transistor and an OP27 op amp. Power voltages are ~ +-14V, just used the positive rail as V+ for the transistor.

(diagram from https://www.allaboutcircuits.com/technical-articles/how-to-buffer-an-op-amp-output-for-higher-current-part-1/)

I verified that the op amp functioned alone in a buffer configuration.

Putting the BJT in the loop added a (negative) offset, though:

WIth a 51ohm load, inputs of 1.5Vpp and 2Vpp respectively:

WIth a 910 ohm load, with inputs of 1.5Vpp and 2Vpp respectively:

I realized afterwards that this circuit theoretically shouldn't be able to go below ground. I'm surprised that the output is basically offset so that the entire output signal is below ground. I guess I will test it again with the input signal being entirely positive, but I'm not sure if this is even related. 
 

Side note: also made measurements of the metal slab (including the tape that is currently around it) to do some calculations later:

34.5mm by 61.5mm (basically flat in the third dimension, plus it's crinkly from tape so hard to measure consistently)