It's usually very important as the power tubes, output transformer, and speaker/cab combo all react sympathetically to create the tones we know and love, though there are a couple of points to consider.
a) Modern high gain amps typically do not rely as much on power amp compression and distortion as their tones are mostly created in the preamp (later Santana through modern metal etc).
2) Vintage style amps usually do rely on their power amp's distortion and compression for creating their tone (UK blues rock through 80's hair metal etc "generally speaking").
It's of course all subjective as all tone debates are, but in my opinion the tube power amp contribution is critical for the types of tones I love (singing mid-gain through pushed mid-gain), plus critically important to the feel in all cases.
The Two Notes power amp sims work great for me, and allow me to have that sound at any volume without the weight penalty that a real tube power amp has (heavy chassis to support heavy high-voltage transformers, and tube power amps can only get the sound you want when pretty much maxed out on volume which isn't always practical etc).
Tube heads are both a tube preamp and a tube power amp together, while of course a tube preamp is just the preamp section alone. Using a tube preamp with the Two Notes power amp sims is a marriage made in Heaven for me (and the cab/speaker sims are great too).
NOTE: In my opinion for best sound with the Two Notes power amp sims start with the presence control at max and work from there.
The power amp modeling is mostly for those of us using stand-alone preamps such as the Marshall JMP1, Mesa Triaxis or Recording, Randall or Egnater MTS, etc, though of course you can use it with anything you like (being careful to properly load a tube head if using it's FX Send/Preamp Out, or Keyboards, Vocals, whatever?). The power amp modeling is GREAT IMO!
I'd check the Two Notes site for cab lists, plus you can use third party IR's as well (such as OwnHammer and Redwirez, without the real-time mic moving feature of course).
DJD- how different in Recording actual use is a good Guitar Tube Pre versus a Head in the final result for singing gain tones?
I'm trying to assess whether we need Power Tubes in the signal chain or not- not enough clips yet to know.
Also no clips of someone playing while someone dials through the Cab IRs on the unit.( ?)