Sound quality impact - preamp vs power amp?

op is a bit confusing, if everything else is high end then both pre and power should be of same pedigree. If funds are a limiting factor then get a better integrated amp that drives your speakers well. 
Consider speakers and power amp as one voice, so if your speakers are demanding (not only power hungry) you need the suitable poweramp, if not, then go for the better preamp.
G

Speaker amp problems are speaker problems. Trying to solve the problem of a hard to drive speaker with an amp is a fool’s errand.

That is nonsense and plain naive. There is more to sound than whether there are "problems" associated with easy or hard to drive. Perhaps MC has never experienced wonderful amp/speaker synergy before. It’s not all about the speaker being hard or easy to drive, it’s about how beautiful is the sound that can be created. Just because a speaker can be driven easily doesn’t mean it’s going to sound good or even the same with amps of similar specs. Why on earth would my speakers sound better with a relatively low powered SET amp than a SS amp with three times the power and with the ability to double down into loads of half the impedance?

I don’t think MC has ever heard the lucid and textured midrange produced by some 300B/speaker pairings while other single-ended triodes will produce more clarity and beauty, and yet others will be richer harmonically if not as incisively quick. All with similar specs and all being paired with an "easy to drive" speaker. MC apparently subscribes to the camp that if a speaker is designed to push more power of out amp, creating louder sounds, then it must be better and will work best in all applications. If volume is all you require from your speaker then your listening priorities are light years away from mine, not to mention grossly unrefined. Perhaps when I was 13 yrs old I would have agreed with you.

As usual, some are answering questions that were not asked, and some are making assumptions.  However, the question as asked may be too open ended to properly address without more detail.

It sounds like the OP is saying that the system is already of good caliber all around.  In that case, a better pre or amp would really offer limited improvements, generally speaking.  If one of those were long in the tooth, or lacked a feature or connection option one wanted, that might tip the scale, as it were.

 I am in agreement with @tvad.  I would like to point out that it’s easier to get a flat impedance curve with lower impedance speakers, and  that due to that lower impedance those speakers will most likely (speakers like tvad’s Soundlabs excepted) be less sensitive.

The amplifier is far more important than the preamp even if it has a phono stage. Amplifiers and how they are matched to the loudspeakers can make or break a system. See what happens when you put an 8 watt SET amp with Wilson's. Some speakers require a lot of power. Without it they are lifeless. Makes no difference what preamp you are using.