New To Tubes Advice Needed

Hi Guys:
Like my question says I'm very new to the world of tubes. I have recently purchased a Xindak MT3 tube Integrated and I'm very pleased with the warm sound it produces in comparison to all the SS amplifiers that I have used over the years. I based my decision to purchase the Xindak based on the sound I heard and not the fact that the unit is made in China. This being my first entry to the world of tube gear cost was somewhat of a concern just in case I did not like the sound in the long run. The unit runs on 4-EL34, 2-68NP (6SN7GT)and one 12AX7 stock tubes original to the unit. I have read that tube rolling could yield some good improvements, since I know nothing on the subject I was wondering if I could impose on some knowledgeable members to guide in so far as to what should be changed and what with. I'm aware that there is so many different tubes on the market and that buying one over the other is a matter of taste but in my case I'm totally in the dark so any guidance is valued.By the way I would like to keep the cost factor in to play so no suggestions for very expensive tubes. Also I have read that EL34 could be substituted with KT77 what is the difference.
Thanks in advance for all of your input.
RossQ