Hey Alicia!
Okay so I am going to be very honest with you there and dont be scared because it might not be the case but, are you sure that its the tubes? I work (worked

) at a music store and we had giant issues with the marshall DSL series because of how they were made.
Again, might not be your case but id rather let you know upfront!
Okay so if we ignore the first part, the only thing you need to keep in mind is to put the right type of tubes in your amp.
If i remember right, the DSL 40c has 4 12AX7 tubes and 2 EL34 Power tubes
Now the power tubes must be a matched pair but luckily they are sold in match pairs.
As for the 12AX7 tubes, they are sold individually so you can just buy 4.
Now as for the brands, some say that there is a difference between brands some says there is not. Thats pretty much for you to see, I wont voice my opinion but I would say that if you would buy the tubes from the same brand, you shouldnt have any issue anyways. I usually recommend JJ or electro-harmonix. Mesa-Boogie are overpriced in my opinion!
Hope I helped!