Hey @apollinaire you are not wrong in looking to upgrade parts inside an amp. Lots of gains to be made at most price points. First off your connection points of RCA inputs and speaker binding posts, check out KLE or WBT. Super low mass designs that are a bit finicky to solder but make great gains in increased clarity right off the bat. Next make sure your internal wiring is super well separated for power vs signal wire. Tight spaces can lead to increased noise that is easy to remedy.
Next up you were talking signal path capacitors, and those could and should be changed but ONLY after you have a baseline what the amp sounds like. As @sns stated, going with AN silver caps could be awesome (and $$$!) they could also voice the amp pretty thin and not what you want. Get some time on the amp and listen for what you'd like to improve. More detail, more body, more transient speed, etc. I'd advise looking at neutral sounding copper film caps as space allows. Some of those can get pretty big 
. Look at Audyn True Copper Max, Duelund CAST CuSn, A.N. Copper, etc. for a very balanced and clean tone. If you need a little heightened treble, you can check Mundorf S/G/O. If you need super snappy transients and clean tone look at VCap ODAM which are great on size as well. Point being, listen to what you'd like to fix and adjust accordingly.
Good luck with the mods!
-Lloyd