Those Mi Si trios get really good reviews, so it definitely should not sound unbalanced.
String balance problems are often due to the bottom of the saddle and the saddle slot having uneven surfaces, so the pick up isn't properly bedded. Check the bottom of the saddle and the surface of the saddle slot for any little bumps - you may be able to carefully sand these. The saddle should sit flat and maybe be just a little slack so that you can pull it clear with your fingers.
In any doubt, take the guitar back to your installer.