The biggest upgrade you can do is buy the R-300B model. It is amazing! I also have the R8 with KT88 tubes and the R-300B is significantly more organic sounding.
calieng3, thank you for this information. Yong Lee recommended the R300B as an upgrade and I have been looking for anything on the comparison of the other Willsenton amplifiers and the R-300B.
I am playing with the R-35i now, but I know, with Covid19 limiting my other hobbies, I will be trying to improve upon it before too long.
My hobby, at this time, is to work at getting the best possible sound (IMO) for the least possible cost.