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

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).

Test Pilot is the only comment above, IMHO, that begins to answer the question.  And to me that is:

What kind of tubes are you talking about and what kind of tube amp design are we talking about?  There are so many variables here.  

The classic tube gear of the 50s and 60s was very tubey and euphonic sounding.  Not the most neutral but that even order distortion was so sweet and lush sounding.  The epitome of tube sounding.

Single ended tube gear in general is a lot less sweet sounding than a push pull design but there's just something so right about the sound.

And PP triode, as written by the late Harvey Rosenberg in Listener magazine, using 4 pin true triode tubes (45s, 2A3's, 300B's, 10's, 211's, 845's, etc) in PP was according to him even better sounding than single ended triode.

I once owned a moded organ amp that used 6BG6's in PP triode and it was one of the finest and lushest sounding tube amps I ever owned and I've had lots of different tube and SS combinations.  Although I couldn't A/B the two amps, my recollection of it was that it was on par, if not superior, to my old McIntosh MC225 amp, which was described in TAS as the best sounding Mac stereo amp, althought the MC240 and MC275 got all the accolades.  The MC225 was Mac's only stereo amp run in triode.

With SS amps, I think mosfet amps sound tubier than bipolar transistors and the chip amps also work extremely well with a tube preamp.

Although its been several years since I heard an EL34 amp, I remember that as being a very lush sounding tube as opposed to an EL84 which was more neutral sounding in the amps that I owned.  

So in conclusion, lots of variables here but if I had to choose between the amp and preamp as which one contributed to the overall tubey sound, I would say the power amp.

The OP question (and more) was answered 40 years ago by Bob Carver.
Short story:
https://en.wikipedia.org/wiki/Bob_Carver
Long story:
https://www.stereophile.com/content/carver-challenge

In brief, Bob Carver Challenge proved that he could emulate any "top"
amplifier with his 400$ (then) model with some pots and circuits to shape the transfer
function. Therefore it’s logical to employ amps as pure "wire with gain" devices and shape the sound to taste upstream.