For tube sound, which is more important: preamp or 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 .

After decades of various enjoyable SS amps, the push-pull ultra linear tube amp I used in recent years (Factory modded Jolida 502P) provided tube sweetness and was, at 65 watts per side, plenty loud. Great amp. After watching a Youtube interview with Dennis Had, I got it in my head that I wanted to try one of his amps so when a 12 watt per side recent version "Inspire Fire Bottle" showed up on Ebay at a reasonable price, I bought it. It shouldn't work well with my alleged 91db speakers (albeit helped by 2 REL subs), and hey…it shouldn't sound much better than other well sorted stuff I've owned. However, it absolutely has more resolution and tonal accuracy than any amp I've owned by a wide margin…is it the simple design? Better transformers? Tube rectification? Class A single endedness? It simply makes me want to sit there and listen to stuff endlessly…things I thought sounded so-so (A recent Nora Jones CD seemed dark and badly mixed on my old amp…but sounds great with this amp…go figure) now sound killer…a cool surprise that really lives up to Had's Ebay hype in the often hilarious little rants that he posts when he sells something.

Ditto to Wolf’s opinion of the 12 wpc Inspire . ABSOLUTLY amazing . But what really blew me away was Dennis ! 6 weeks ago , I was playing a new matches pair of Gold Lion KT-77’s ( 150 hrs ). I heard a pop , then another , fairly mild . Shut my stuff down and went to bed . Next morning , I switched the 2 tubes, installed a different driver and rectifier . Turned on my system and just let it sit there with the CD on pause. About ten minutes in , a pop in the other Chanel . So as I watched the amp and contemplated a bad tube , Kaboom ! At about 100 dB. Dove for the switch . No light show , just a trickle of smoke from the chassis . Pulled it apart and I had completely destroyed a couple caps , a resistor with such force that it snapped the terminal strip . I emailed Dennis , HELP ! 30 minutes later my phone rang . He said ship it to me and I’ll fix it for free . I said it’s 18 months old and it was my bad tube . He said , It never happens and I’ll fix it or build you a new one for free . I shipped it to him and it was home , fixed and return shipping for free in two weeks. I begged Dennis to let me pay him . He reluctantly said OK , 100 smackers ! How many times have I met a man like him ? Not enough . All he cares about is the Audiophile enjoying his music . Try his amp . Like I said on his EBay feedback - BEST 12 Watts I’ve ever heard ! Then I went to a lower rectifier and the KT77’s and Heaven . Imagine what top notch Mullards would sound like ! I do have NOS 6550 Tungsols , amongst 5 other pairs of various values . And the tube selection for my pre ! Same results with the single E88CC Tube in the gain stage of the Inspire . Flipping Sweet ! It would sound amazing with a good pair of small speakers and stands , and subs ! 

I have heard a recent new product from a well established company with very innovative approach to class AB amplification in my system for a 30 day trial. Incredible resolution. Super clear, dynamic, great soundstage...etc. but after some time too clinical; almost too perfect to the point where my brain disengages from the sound and I want to walk away. Too "high fi". Could not relax with this sound.This is NOT a direction I want my system to go in. I sent it back.

Benchmark managed to make a Class AB amp sound like a Class D amp. Quite an interesting accomplishment. Suck out the distortion with NFB and suck the life out of the music. There is no free lunch with NFB.