For tube sound, which is more important: preamp or power amp?

If it were me I would go for a good tube amp. One that is not fat sounding. That can also be caused by the tube amp / speaker combo. so make sure that the work well with each other. 

So now that you are there you can try some different preamplifiers, both tube and solid state and find out which one you prefer. 

I have been chastised for not indicating that I am a dealer and I just forgot to do that. Sorry. 

Many meaningful contributions presented.  In my experience I have discovered that finding a great preamplifier a daunting task. Let's face it, if your preamplifier can't resolve the micro-details/dynamics along with the other obvious positive qualities, you have already lost the battle. The power amplifier can't compensate for lost or inaccurate information. Seems easy enough but in reality not so much. 
In this light I would place the preamplifier as the most influential electronic component in the audio chain (limited to preamplifier & amplifier).
Remember that our passion and pursuit of our individual goals is not assisted with implemented standards by manufacturer's that helps us along our journey. 
No standard voltage requirements to drive an amplifier to full rated output and impedance issues as well. Many preamplifiers are fussy about length of Interconnects, etc.
Best of luck and happy listening!

I have tried all combination's with exception (by choice) to having both the pre and amp being tubed.

I find for low level, late night listening, a tube amp or integrated amp will sound much better than anything else for the money.

A tube preamp is useful for injecting just enough 2nd harmonic distortion to help ameliorate much of the phase issues or crossover distortion with lesser quality SS amps or Class D amps.

I found that listening DAC direct (balanced) to my class D monoblocks to be pretty much unlistenable.. grey and clinical with an over-emphasis on detail and HF transient response. However i've heard worse for much more $ at trade shows, unbelieveably.

Because of my speakers (86-89db depending on reviewer), I use a NOS VT-99 preamp with class D monoblocks. This gives a very listenable sound with a relatively low noise-floor, and i can crank it up if need be and the soundstage stays coherent and fleshed out.

I also have an older but still excellent Pass Aleph amp which may not have the detail of class D with a tube pre, but driven directly from my DAC, it (despite being only 30 watts) has a very full bodied and spatially delineated sound. It sounds organic enough, or 'real' perhaps, even more so than tube pre to Class D. This is true for most my music except for for tracks with samples of rainfall, where the individual rain drops are slightly smeared compared to Class D with my tube pre. I assume this is most likely due to the Aleph being an old amp, older technology, running in Class A, but more importantly for me, it still doubles down into 4ohms. Maybe a Pass 30.8 would be an upgrade to the old Aleph.

The tube Pre paired with the class A Aleph, sounded too soft and sinewy for my tastes. Direct from DAC was more 'right.'

I use the tube pre and class D for summer listening, and the DAC direct to Pass for winter.

One other consideration is it took me quite a lot of trial and error to find the right tubes for my pre. Maybe $3,000 invested in tubes total, but once i found the right ones i loaded up because their difference is just that great.

let me let me append all that with the answer to your Q.

The amp will give you the most tube sound. Preamp is merely seasoning (although sometimes the seasoning makes for the best soup).