I have the Line Magnetic LM-508IA and tubes made a big difference in sound. In fact, power tubes (805 in my case) also made a considerable difference, may be even more than preamp tubes.
If Im not wrong, you said that your amp has 50 to 60 hours of use. I would burn more the amp as I believe its not enough to judge its performance. In fact, during burning time, one day may sound worse than the day before. I would burn it at least for 300 hours before changing anything.
Then, if you want to roll tubes, I would start with the preamp ones, wich are cheaper and can give you an idea of the changes.
By the way, Ive tried changing the original fuse with a HiFi Tuning Supreme one and Ive found an improvement, but not huge. I would start with preamp tubes after 300 hours of burning with out a doubt.