Preamp or power amp has more impact to resolution?

I believe your amp is your limiting factor just not enough power to drive your speakers properly. Good Luck.

There is always one (big) thing you can do about preamp/amp resolution downstream...power conditioning. Elizabeth is quite right about that.

Over many years, my experience has been the priority ranking is Linestage first, then Power Amp, then Phono stage. Obviously you don't want to get too far out of balance by, for example, pairing an entry level power amp with a top-of-the-line linestage, but in general the linestage seems to have the greatest impact on overall sound quality. I have had a number of phono stages and power amps that I find quite good sounding, but very few linestages that I have actually liked.

Well said Salectric - that is my conclusion as well on all points.

...which upgrade that can provide more impact...

... at the end of day: the Quality of your Speaker Drivers and the sensitivity after
the X-over