If it has a tubescreamer attached to that switch
Okay I guess the DSL40 doesn't have gain then?
Actually it has tons of gain. There are two channels, Classic (low gain) and Ultra (high gain) that are foot switchable. Each channel has two gain modes. The Classic Ch lowest gain is clean clean. It runs through a fixed Fender black face tone stack built into the signal path in this mode. The other mode is crunch. The classic channel crunch mode really takes boost pedals well. Many only use the crunch mode with pedals all the time as a one channel amp. The Ultra channel has Lead 1 and Lead 2. Lead 1 has plenty of gain for classic rock and 80's metal. Lead 2 probably has too much gain for many applications, but it's there if needed.
Last year Marshall redid the DSL40 calling it the DSL40CR. On the CR there is a foot switchable extra master volume, and the modes can be switched within channels with a midi controller if needed. The entry level speaker was also upgraded to the Celestion V type.