Thankyou, it is indeed a complex thing. I thought the same as you purely by reasoning that I read somewhere Line6 only modelled the Hi inputs, and advise to use a vol/gain reduction block to imitate the low inputs before the amp.
This meant if you did "jumper" the two remaining jacks there would no 3rd jack for the guitar, lol, so it must be modelled via a split Y cable into both High jacks :)
Having done some listening this afternoon comparing preamps to the SV20 jumped as above and using the Normal Low guitar input, the Helix Plexi jump and bright preamps are not too far from my SV20 but are darker in character, less sparkly.
The JTM45 is even closer, but the standout winner is the Who Watt, near enough spot on.