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

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.

   , that’s a Big Question . It feels like I’m trying to arm wrestle an octopus 🐙. So in regards to just your present system . I run a modest price Rogue RP-1, that was a cost driven purchase . This pre uses a pair of 12au7’s . I also have quality interconnects . What I have found rolling tubes in this amp and in regards to your system and musical preferences is this . I can change the entire presentation of the system by changing these two tubes. NOS are my go to BTW. If I run Mullards it’s soft and mild , with the entire soundstage subdued. If I run Amperex Holland , the overall is very bright and forward , losing bass ( not the fish ). If I run Seimens nickel plates the top 1/3 is forward of a fairly full sound . If I run Telefunkens , my porridge is JUST RIGHT . So my opinion is,  if you go tube preamp you can play with tone and the layering of the music ( depth ). This has allowed me to fine tune my system , especially female vocals . If I listen late at night I can enjoy Diane Krall as if it’s live. I also like Natelie Merchant and Amy Winehouse . BTW , my wife put me on Simon and Garfunkel probation . If I was to get a new pre today , I’d consider Don Sachs . Advise from Wolfie and Whitestix is aligned with mine . It’s just that we have similar tastes in music . Everyone that posted has found a good Synergy with what their taste requires. Now that I’m through promoting, recently I blew up my Dennis Had Inspire Amp . While it was off for repairs, I ran a Vintage Marantz SS amp . I had a ton of hiss . Tube pre with too much gain coupled to a SS amp that requires low signal into a 98db speaker equals MUCHO HISS . It’s all about compatibility ! Back in 1975 ( I’m old too ), I ran a SS pre into Dynaco MkIII’s to Jbl’s . It was fun . As far as el34’s , they’re great ! I really like the new Gold Lion KT-77’s and the SED Winged C’s in EL34 and 6L6LGC . As far as the Schiit pre , I love their stuff , but it’s a compromise and you’d be better served going full tilt on a pre . BTW , I’m saving my penny’s for a Gumby or a Yiggy , I have an Uber Bifrost . Good luck on your endeavor, it’s only money and sanity .