I defer to Raul re: silver vs. copper wire, since copper is all I've heard. (One exception: copper coil ZYX's are more neutral than silver, and silver is more neutral than gold. This is not what I expected before I demoed them, but it is what I heard. Others have heard the same.)
Forget Koetsu. Every one I've heard (up to the RSP) was colored. Many people love the Koetsu sound, but I was always concious of hearing the Koetsu, rather than the vocalists or instruments (especially instruments). The main characteristic is about the opposite of the 901. Transient leading edges are rolled off and smoothed. This makes the sound artificially pleasant, warm or sweet. (The silver and gold coils ZYX's do something similar.)
The regulars here all know what I'm going to recommend. Buy yourself a ZYX UNIverse-X-SB. It is a perfect match for your arm, a perfect match for your sonic goals, a perfect match for your musical tastes - all of which I share. Comparing it to a Shelter 901 would be ridiculous. The UNIverse is unimaginably superior. A ZYX Fuji, Airy 2 or Airy 3 would meet your needs better than a 901. A UNIverse will probably meet your needs for as long as you listen to music, or until you wear it out.
Please read the reviews linked next to my signature. They will give you many details.
What I can say is that I want a sound wich is the more natural as possible(NO COLORATION at all) with the more details as possible ( no agressivity, no HARSHNESS) .Well then, the good news is your present setup (901/TP) can be improved on a great deal. The 901 was slightly peaky on every arm I heard it on (except a Schroeder Ref). On a TP VII, Graham 2.2, Basis Vector and modded OL Silver it always overshot on leading edge transients. This gave it a lively but artificial character that would sometimes sound aggressive or harsh. Once you've heard a truly neutral cartridge a 901 becomes almost intolerable.
I am listening to 60% classical and 40% jazz .
Forget Koetsu. Every one I've heard (up to the RSP) was colored. Many people love the Koetsu sound, but I was always concious of hearing the Koetsu, rather than the vocalists or instruments (especially instruments). The main characteristic is about the opposite of the 901. Transient leading edges are rolled off and smoothed. This makes the sound artificially pleasant, warm or sweet. (The silver and gold coils ZYX's do something similar.)
The regulars here all know what I'm going to recommend. Buy yourself a ZYX UNIverse-X-SB. It is a perfect match for your arm, a perfect match for your sonic goals, a perfect match for your musical tastes - all of which I share. Comparing it to a Shelter 901 would be ridiculous. The UNIverse is unimaginably superior. A ZYX Fuji, Airy 2 or Airy 3 would meet your needs better than a 901. A UNIverse will probably meet your needs for as long as you listen to music, or until you wear it out.
Please read the reviews linked next to my signature. They will give you many details.