Indeed, the Soul is an excellent analog preamp, the best I have heard or measured.
If you have at least 6 dB of headroom with the Soul MK1, then you can get an idea of what difference the MK2 might make. Set the Soul's digital gain switch to the LOW position. This drops overall levels by 6 dB, which unloads the DA converter. Then turn up the volume accordingly (about 8-10 clicks) to restore the level to the same as before. The effect won't be fully the same as the MK2 conversion, but it gives you a small taste of what MK2 does.
This won't make any difference with analog inputs, but I don't think MK2 makes any difference with analog sources either?
@Jan Meier please correct if that is wrong.