Sound quality impact - preamp vs power amp?

Depends on what future plans are. If you put more into one than the other do you plan to upgrade the lesser soon. If this is going to be more long term setup i would say get an amp and pre-amp that match well. If you get one higher quality than the other then you will not get the most of the higher end component.

I don’t know.

What are your current  sources?

Is the preamp/amp solution your last upgrade?

I have upgraded my system in stages and each step was a noticeable improvement. 
I did the upgrades based on a goal and took advantage of sale items of demo equipment. 
I upgraded from a purely analog system in several stages:
1.  Bluesound Node 2i. Adding digital streaming to a purely analog system. 2.  Added a preamp with a phono stage and DAC and used my integrated amp as an amplifier. 3.  Then I bought a stand alone amp
4.  Then I bought a new standalone DAC. 5. Then I upgraded the speakers 
6. Then I upgraded the turntable 

if I did it all at once - I would consider an integrated amp. There are many good ones available. 
I don’t know if you listen to vinyl, CDs, streaming or a combination of sources. 
Trust your ears and determine if your system is a journey or destination.  And good luck 

I chased this power pre perfection and was never really happy
Had an ATC P1 recommended stereophile had tron and croft valve pre and harbeth speakers... Good but something missing. 

Eventually got my speakers sorted.. Tannoy which matched with what I loved and then Luxman class A integrated and perfect synergy.... 

Maybe if u had time and a lot more money it could be improved... But I just want to enjoy the music. 

I upgraded in stages as well and I think I have a good dialed in synergistic system. I would like your opinions. Bob Latino st-120 amp, Vincent sa-T7 pre, music hall cd/dac, bluesound node 2i, thorens td 125 w/Hana eh. Bob Crites cornscala speakers with wireworld oasis 8 speaker cables and interconnects. Furman power conditioner. I think it’s a modestly priced system that punches way above its weight class. I figure I’ve got about $10K invested all together.

I have a tube preamp and amp that works well with my speakers. They have good tubes.

I have a good integrated solid state amp that works well with my speakers.

Here’s the comparison I did. Keeping the DAC and the source the same:

(a) SS-Tube. When I run the solid state integrated as a preamp only through the tube amp, it sounds ok.

(b) Tube-SS. When I run the tube preamp through the integrated’s amp section only it sounds much better.

So, in my particular setup (only!) I can conclude that the preamp makes the bigger difference.

My tube and solid state are at similar price points. I realize that my tastes, impedance matching, etc. could be factors here, but I do hear a clear difference. Preamp more important in my setup.