Tube amps - what 3 things…

• Auto-bias circuity (this applies to push-pull topologies) is lovely, and usually includes superior protection & status indicators that are preferable to fuses. I’ve had a tube fuse blow and it wasn’t even immediately apparent that single tube was out (multiple push-pull pairs per side). I’ve also had a fuse fail to trip when a KT90 failed "spectacularly" and fried a resistor (and singed the PCB).
• These days it might be wise to choose an amp that DOESN’T lock you into future Russian tube supply, e.g. KT90, KT120, KT150, KT170.
  KT88 are nice, reasonably high power, and have non-Russian alternatives available.
• You might first decide what power level you’re looking for and branch from there. Low power opens up options like DHT (direct heated triode), SE (single ended, aka pure class A), and SET (single-ended triode - usually SET and DHT go together, e.g. 300B tubes). Medium power can be hit by either big DHT tubes or medium size tetrodes / pentodes like EL34 and 6L6GC. Some say those tubes still sound sweeter than the big tetrodes like KT88 - KT170 (and I’d tend to agree). For high power, you’re looking at class AB push pull, KT tubes 88+, and (often) multiple paralleled push pull pairs.
• Amp driver circuits are often overlooked and definitely affect performance. I’m not a fan of 12AU7 here. I think 6SN7 is a lot better, for starters.
• Always look at the service history for a brand. You want a brand that takes pride in its products and will service old out of warranty units without either balking or acting like they’re doing you a huge favor. Ideally, they will be based in your country. And really ideally, point to point wired amps are generally more serviceable (especially in the long run) than PCB based.

I second the auto bias feature is great to have,flexibility in which tubes to use,and a gain control and feedback control are nice features. Point to point wiring is a big plus - simple to upgrade parts and repair if needed.

Thanks all. @jjss49 I deliberately left out that detail as I was looking for a 1st pass on features. But understand your point. The listening space is an open plan 7m by 9m. There won’t be any decisions without home demos. 
 

@mulveling this is great information- I really love the point about out of warranty service and auto bias. I have an allnic in the house now and need to adjust bias every time I switch it on - is this normal?

I have an allnic in the house now and need to adjust bias every time I switch it on - is this normal?

@mpoll1 No it is not. Are you allowing the amp to warm up for 15 minutes or so before checking bias? Bias should be pretty stable once it is set. As tubes age it may need to be adjusted, and at a certain point they they will need replacement if you cannot get the amp to bias properly. I prefer amps with fixed bias vs. cathode self-bias, but I have access to a computer tube tester and a huge stock of tubes so I can ensure well matched tubes for use in any of my amps.

The most important features depend largely on the buyers attitude and expectations regarding components.  If you are someone who is bothered by having to maintain/fix gear, or are bothered by the fact that tubes start to go bad from the moment they are turned on, then tubes are not for you.  To the extent you have some tolerance for the nature of tube gear, but, you are still concerned about tubes wearing out, that might affect the kind of gear you should buy.  The type of tube chosen, and how hard the particular amp drives those tubes, are BIG determinants of how long tubes will last.  There are amps that will run for many decades without the tubes having to be changed, others might need new tubes every couple of years (or earlier).   Tube amps tend to be more noisy than solid state--hiss or hum or intermittent crackling, etc.--you need to factor in your personal tolerance in deciding what to buy because some otherwise great sounding gear might be prone to be noisy.

For all kinds of gear, not just tube amps, decide what features are necessary.  To me, remote control of volume and some form of balance control is essential.  Given the nature of tube gear, I would think that small channel imbalances are inevitable and the ability to compensate with a balance control is a must.

The world of tube sound is vast and very variable--much more so than the sound of good solid state.  You need to listen to enough gear to determine what fits your ear.  Personally, I don't think there is only one type of tube topology (e.g., single-ended triode) that is clearly superior to another, but, there are some listeners that have a clear preference.  I have heard versions of pushpull pentode/tetrode amps, single-ended triode and output transformerless amps that sound terrific, often in quite different ways.  I think the type of tube, and the brand of tubes matter too, but again, these are personal preferences so the only recommendation is to hear plenty of different examples to develop your own idea of what to buy.  Finding a friend and/or a friendly dealer that can help you sort out options would be a big help.  If you attend a big audio show, you can also taste different gear or find people that can help you make the right choice.