My amp so like crap after being fixed?

I own a marshall JCM 2000 dsl 100 and had to change the tubes in the amp because they wore out on me. When I got it back the man said that he put the same tubes back in and everything. But when I plugged my guitar in at home my tone said all scratchy with absolutely no midrange! I even had the mid turned all the way up! I was just jamming to try to make the tone a little betting and I noticed that there will be slight volume fluctuations as well as the mids coming in and out! what is wrong with the amp?!