It sounds like you are still getting to know your amp? You may want to give it a little time first and then roll some NOS or new tubes first and see what you think.
Some integrated amps can be enhanced by using a nice preamp. The Jolida's are a good example of that. They are basically an amp with a volume control. I'm not sure if that is how the Cayin is set-up? Anyway even if you add a preamp you will be using most if not all of your tubes anyway in your amp anyway. In my experience tubes will make a big difference.
I truly don't think you should give-up on your amp before seeing what it can do with some nice tubes. If you do add a preamp stay with tubes. Just my two cents
Some integrated amps can be enhanced by using a nice preamp. The Jolida's are a good example of that. They are basically an amp with a volume control. I'm not sure if that is how the Cayin is set-up? Anyway even if you add a preamp you will be using most if not all of your tubes anyway in your amp anyway. In my experience tubes will make a big difference.
I truly don't think you should give-up on your amp before seeing what it can do with some nice tubes. If you do add a preamp stay with tubes. Just my two cents