I cannot totally agree with the above comments.
In the past, I have used my 'Les Paul' solid electric guitar through a Technics Keyboard (KN6000) and found that using the Line Input did not produce sufficient level. I transferred the connection to the mic input and it gave perfectly satisfactory results - albeit with the cautions mentioned above, about initially keeping the guitar volume controls low.
I have just tried connecting my guitar to the mic input on my KN7000 and it works equally well and is certainly not 'tinny' or 'weak'. The set-up produces a good rounded undistorted sound with plenty of body. The KN7000 MIC slider was set about 1/4 way up and the guitar volume controls to around position 6 of 10.
Using the Line Input on the KN7000 did not produce sufficient volume level with my particular guitar, which I believe has fairly standard pick-ups.
I guess it's a case of 'suck-it-and see'
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Willum