Marshall DSL40CR arrived, is this correct?

[ErikH]

If you want to get more range out of the volume knob, if the FX loop does not have adjustable level, is put a pedal in there that cuts the level. Most pedals with level at noon is unity gain, so you can reduce output. EQ pedal is good candidate for this as its always on.

A “volume pot in a box” is perfect for this. They are all over eBay. Slap it in the effects loop, turn up the master to 4 or so, then choke it back with the volume box in the loop. I used one with the 100 watt Marshall head I had.

Note: It is not an attenuator. Never never never put one between the speaker out and the speaker. It will kill it and probably the amp too. Effects loop use only.

 

[ErikH]

This. I have built these, and they work very well, but they are more like the volume control in a guitar than anything else, and they are meant to work on low-level signals before the power stage of your amplifier. It is a master volume knob that can only turn down (not up) the signal after the preamp and before the power amp section.

They are very helpful when the master volume of the amp is very touchy, such is the case here. The same with the Fender Hot Rod Deluxe amps. One might be too quiet but then you get to two and it’s just too loud. Rolling the guitar volume back isn’t going to fix it. Putting one of these volume boxes in the effects loop will. Rolling the guitar volume back is only changing the signal that is hitting the front of the amp. The effects loop is further in the circuit, usually after the tone stack and before the master volume, so you’re still able to get all of your full preamp sound that you have dialed in but with a better master volume control.

 

[JMP/HBE]

I think Mike was right, Marshall is clearly up to no good here as he suspected.
Better send it back.
........ jk
For those of us that got our first tube amps 50 years ago there was no internet and trial & error could be costly or even catastrophic.
And asking questions is a sign of intelligence but i suppose keeping things in balance applies to internet chat forums too.