Strange tone Marshall DSL40C

The problem is caused by a microphonic tube. Tap on the tubes while the amp is running and it should not be hard to find out which is causing it.

If the amp makes a sound no matter which tube you tap, try just rubbing the top of the tube gently- that will make it easier to find.

I confess that I have  Fender Twin Reverb and a Mesa Boogie! Plus a Laney 100 watt tube head and 412 cabinet!

"I've got a fuzz box and I'm gonna use It!"

This distortion you are hearing could be oscillation caused by circuit negative feedback interacting with a limited bandwidth output transformer (OPT). Tube amps are very sensitive to the amount of negative feedback applied. Less is better! And a better OPT (wider bandwidth) would help! Guitar amps are notorious for having poor quality OPT's. Cheap and distortion-prone! Then again that's what electric guitar players want- DISTORTION!

Is this a combo guitar amp? Having the tubes and circuitry in close proximity to the drivers (speakers) is BAD! Subjecting tubes to high levels of sound waves and vibrations is BAD! Internally tubes are rather delicate devices. Prone to acting as microphones and "feeding back" when subjected to vibrations. Better to have a separate amplifier "head" placed nearby away from the speaker cabinet!