Marshall makes a variety of amps, with a variety of tonal characteristics and gain stages. They all retain that Marshall sound, but with some models the higher gain stages can sound a bit compressed (think “JVM series”). The more open, organic models (like the JCM800) feel very natural and responsive but need a little help getting into high gain territories.
A lot of Marshall owners have turned towards modifying their lower to medium gain Marshalls to get more distortion out of them. There are plenty of common mods available out there (including no-drill mods for certain models) but there are also a few guys out there pushing the envelope when it comes to Marshall modifications.
Hermansson Amplification is a company that is doing some absolutely killer work in the Marshall modification sphere.
Just listen to the example below. The Marshall 1959HW is modified with 3 gain stages and a tube compressor circuit, as well providing four foot-switchable options:
Another great example is this JCM800 2205 modification.
Here he adds an extra preamp tube and distortion channel based on four gain stages.
While many of us will boost our Marshall’s with overdrive pedals or even add a distortion pedal to really drive the organic sound of the amp, an amp modification is a way to get all the distortion you need right in the box.