I would like to know this also, seeing as I'm getting a new amp soon.

new tubes can last for years without replacing...it all depends on how many miles you put on them. constant play say 3 to 4 hours a day and you may have to replace them after a year or so.....in other words, they last a good long time. you shouldn't need to take extra tubes to a gig....the time for standby in not a big deal either. you can fire it right up without standby if you choose. standby is nice for quieting the amp between sets or longish periods of time. you CAN change tubes on your own and there are a bunch of sights with information on the process but...it's never a bad idea to just let a tech do it. and certain tube amps have a fixed bias(where replacing tubes is all that is required) and certain ones do not(where some biasing may be necessary). the real problem is that most of these amps have high voltage parts in them that can HURT you if you aren't very careful....even when they are unplugged. so taking it to a professional is always a good thing to do. but realize that the majority of tubes out there have a pretty long life span so this isn't an operation you will have to do very often.

Oh so close but no cigar. You're right tubes can last forever but generally with 4 to 5 hours a day or 3 to 4 4 hour gigs a week,about a year for pre-amp tubes is about average. YES carry a couple of spare pre-amp tubes always. Bachmirage has obviously never had a tube fail an hour into a gig rendering your amp nothing but a large expensive paperweight. Believe me it will happen and the MOST inopportune time you can imagine. Also carry a couple of extra fuses.
Standby is there for a reason. Best practice says, turn the power on with the standby on and let the amp warm up for about 2 minutes, then turn the standby off. When turning off the amp it really doesn't matter.
Can you replace tubes yourself?? yep you can replace pre-amp tubes all day long. Turn off the amp, unplug from the wall and just pull out the old and put in the new. Power tubes are a bit more tricky, Bach is right if you have a fixed bias then you're probably all right just replacing them however; if you need to have the bias adjusted then it's time for a tech. Bias adjustment is not a hard thing to do but, this forum is not the place to get into it. I honestly can't remember if your AC30 is fixed or adjustable bias. Best advice is RTFM. (Read the F&*%ing Manual).
Now having said all that; it will take some playing time for your amp and speakers to "break in". The more you play it the better it will start to sound.
Most important in all this is enjoy your new amp. I have played a couple of the hand-wired AC30 re-issues and it seems the new Vox amps are pretty cool pieces.

i never DID have my tubes go at a show. i used my marshall jcm900 for about 4 and a half years before i had to replace any tubes( i realize that was pushing it, but if it works,then what the hey). and even then i only did it becuase i figured it HAD to be about time. of course the only time i used it was for actual shows, and there were plenty of those for sure but i didn't used it at home. i just used one of the other amps i have or my pod for practice. i suppose it wouldn't be a bad idea to take tubes to a show with ya, but more convenient is simply having a back up amp or pod for that instance as you may have biasing issues with said amp as was stated allready. actually, having extra FUSES for your amp is important as well. i DID have a fuse blow at a show which is why i had it re- tubed anyways. once you start blowing fuses, you know you got problems with the circuit anyways, that said, you can see there IS some maintenance that goes along with a tube amp but it's well worth it. nothing against solid state's(i've used many for live shows) but there's nothing like the warmth of a tube amp. also being able to simply roll the vol. knob down to get your clean sound is a nice feature as well. you really can't get that same clean roll-off effect with a solid state. i've seen debate about the standby go either way...let it warm up first or don't worry about it at all.....i personally haven't had any issues with that one way or the other. but more often than not, you will be turning it on and letting set for a few minutes anyways so technically it's probably good to let them warm up a bit. they will start to sound a tad better after they warm up for a while. not a HUGE difference, but enough to notice somewhat...

you'll also be pleasantly surprised at how well foot pedals work with your amp. especially overdrive and distortion pedals. and when you do go to eventually change your tubes, try to stay with the same tubes that came with it.

oh...and reading the manual is excellent advice...know all that you can about it..........

also don't leave your amp on standby for long periods of time.

When you have standby off and when playing there is a voltage differential present between the cathode and anode in your tubes. Even when not playing you have a biasing voltage present on the cathode-anode junction. Normally the heater element in the tube heats the cathode allowing electrons to be emitted from the element into the vacuum in the tube. Since only the cathode is heated only it emits electrons and the electrons are drawn towards the positively charged anode. This naturally causes deterioration of cathode which is why tubes wear out. Standby keeps the heater on to keep the cathode warm but removes the voltage across the cathode and anode. Therefore don't leave your amp in standby for more than you need to, turn the power right off if your not going to use it again for more than 30min (heating and cooling the tubes repetitively also wears on the heaters and tube body so don't turn it off between songs, but maybe in between sets; most importantly don't forget your amp on and leave it for hours or overnight!)

Alway turn the amp on standby before turning off the power switch, saves you speakers from pops caused by collapsing bias circuits.

Also, don't let your tubes get too hot. Tube amps have vented grills and usually vents on the top and back for a reason, the tube requires a temperature gradient between the cathode and anode and if the anode gets too warm you can have electrons emitted from the anode which decreases your gain. as well overheated tubes can melt the heater filament rendering it useless. Your amp will stay cool enough as long as you don't cover up the vents, so don't place anything on top of the amp (especially drinks for other obvious reasons) and don't set it right up against the wall.

read the manual, the answers you seek lie beneath the cover, my young padawan lol