Any technique / Sonar plugin to match guitars?

Based on the limited info, if the sound is too bright, then the previous cable probably had a much higher capacitance/foot spec. Try using a coil cable, they have much higher capacitance/foot.

brian brock
fix the old cable!

This is probably the best way to go.

If the amp sim has a presence control, try reducing presence rather than treble. A poor quality guitar lead not only rolls off treble but it reduces the signal received by whatever's attached to the non-guitar end of the cable. So there might be a need to reduce the sim's gain/distortion somewhat as well.

The "pros" handle this kind of thing by either picking leads that are good quality (which for guitar means one with a low self-capacitance and low handling noise) in the first place, or by sticking to the same or very similar cables. Sometimes a "poor" audio quality cable, such as a coiled one, might get picked precisely because it reduces treble and takes off a bit of gain - can be very useful when dealing with a Telecaster or Rickenbacker into a bright amp, but that's where sticking to the same make/model of cable becomes particularly critical.

EQ alone would probably do a lot, but hard to tell what the A/B comparison is.

A simple technique I use most is pan/loop a reference left or right and the guitar in the other speaker. If you are between them, you can tweak the guitar to match the reference.

There are also EQ matching plugins which will do similar but more precisely (provided you have the reference), but I am not sure of a good one that is free.