Tube amps - what 3 things…

From what I hear DHT is right, but budget and such matter. Rule 1 though is GIGO, garbage in, garbage out. My system with an old PS Audio 200C or Acoustat 3l200 watt amp would sound good, but my Mark Levinson smokes them both. OTOH, if I had a $100 Schiit DAC in my system it would sound far worse than a TNT in it with the Lampizator DAC. The thing that I like though may not be what you like, so if I give you the correct answer it's dumb luck that we have similar taste in what we seek from music.

A lot of good advice from the group here, although it doesn't necessarily square 100% with my experience, especially those who commented on impedance issues, tube life, and noise.  I have two primary systems designed around the use of high and low-medium efficiency speakers, Altec 604 and Magnepan 3.6.  I use tube amps from 1.5 wpc to 35 wpc with the Altecs, and tube and SS amps with the Maggies, tube preamp in both cases, and power output from the mid 50s to 120 wpc.  The two SS amps I have range from 150 - 200 wpc.  They all sound great because they're the keepers of the dozens of amps I've vetted. 

The key is choose your speakers first.  Then let the speakers drive the amp options.  My tube amps don't care if they're running 4 ohm or 8 ohm loads, have had both, and the 4 ohm loads do not cause any issues with greater loading, heating, etc, with the tube amps, but yes, the SS amps run hotter with the 4 ohm load.  As far as tube life, I run primarily old stock and NOS tubes and since I only listen seriously a couple hours a day, they seem to last forever.  I can't recall the last time one of my tube amps was noisy for any reason.  With the Maggies' lower sensitivity,  tube amps are dead quiet.  With the Altecs, there's a tiny bit of hum with the SET amp, and the rest are nearly silent.

Also great advice from those who said to avoid amps with proprietary or single source, or otherwise rare tubes.  If an audition with your speaker choice is possible, then you'll be sure you're getting the best matchup for your audio investment.  Have fun shopping!

For me autobias isn’t critical as long as it’s easily adjusted. I’ve owned Unison, Allnic, Quicksilver and Audio Research. My favorite sounding was my AR which is a pain to bias but the others provided an accessible dial. Biasing once tubes are settled should be monthly. Stick with a company that has been on the market for a while and has a solid support network. Buy an extra set of tubes or two and you should be set for a decade provided the amp isn’t left on. Good tube amp hygiene, no turning on and off in quick succession and allow a 15 minute warm up or more before hammering the system.

Checking bias monthly is OK, but knob dicking the adjustment pots will shorten their life. Tweaking for the last fractional mA is pointless. Great amps were biased with moving coil voltmeters for decades before digimania took over.

The bias is going to shift slightly with line voltage, time on and ambient temperature variations. Home users would do better to check bias at the end of the listening session with the amp and components are fully warmed.

I have to say that I am enjoying this thread alot, and I am learning quite a bit, being a newcomer to tube amps (I have owned my first for a couple of months now), I get the feeling this rabbit hole runs deep!