If you are not familiar with Marshall amps in general, then you should understand that (a) Marshall amps tend to be a little darker than say a VOX or typical Fender amp, and (b) Marshall amps are best used at higher gains, not necessarily louder volume, which makes their circuitry work harder with better clarity. There is also the speaker response. Lower volume means the speaker is not working as hard as it could, or should. My experience has been that JVM800 and DSL series are darker and could sound muddy compared to a VOX or Fender amp. I use a Strat and have found that the JTM45 settings are crystal clear and quite bright. If I change to a Les Paul, or something with a humbucking pickup along the same lines, the tone can get a little darker. Check your tone on the guitar, or adjust the treble on the amp. Try increasing the amp volume and lower the master volume when using a Marshall series setting. Presets with American in their name will work better with a Fender style guitar, if you are looking for a very bright tone. As you may know, you can control a lot of the tone and amp response with the guitar volume controls. Another thing to check is the height of the pickup relative to the strings. The pickup may be too low and you're not getting enough signal.

Just a couple of things to look at. Mostly, it's trial and error.

Hope this helps.