#1
After much delay, I finally got myself the required tools to connect my guitar to my PC and not have it sound shit today. I've been playing with Amplitube and I'm getting awesome sounds, and while I have been GASing for a Blackstar HT-5 to replace my old 15W SS amp (...AASing?), I feel like I don't actually need the Blackstar anymore. I believe there are differences between a sim and an actual amp, but having never played high-end amps, I honestly cannot tell much difference between a sim and an amp.

Off the top of my head, the main problems with using my PC as my main amp is I'm using headphones, and I'm not about to get some speakers just to act as a mini cabinet.

I'm also only able to use Amplitube after turning off all other sounds, so unless I import an mp3 into Amplitube, I'm not about to be able to play to a backing track or a studio recording.

I have absolutely no plans to play at gigs, but it'd be nice to let whoever I know hear what I'm playing without having to pass them my headphones.

Is it common for people to just go all digital?
#2
A lot of people go all digital for the conveniency, but they usually go for Kemper's or AXE FX's.
I have some considerations about your post:

1. the HT-5 is not a real all tube amp, it's not a really high end amp, the emulated output sucks, and my personal opinion about the sound is that it's pretty bad, too.
By any means get one if it's what you feel like needing and if you like the sound, though if you have any problems with any of the above, don't get one.

2. amplitube is not really regarded as a that-good-sounding amp sim, so before really settling for that I'd try some other stuff.
bx_rockrack, softube amp rooms, LePou's stuff, Nick Crow's stuff...
But then again, if you prefer amplitube, by any means use that over everything else.

3. the biggest differences between an amp sim and an amp are the conveniency and the way in which they process the sound.
You know, the analog/digital thing.
But good amp sims sound good, and they are more practical for lotsa stuff, so people use them.

4. "I'm also only able to use Amplitube after turning off all other sounds, so unless I import an mp3 into Amplitube, I'm not about to be able to play to a backing track or a studio recording."
That wasn't very clear.
Are you using amplitube's standalone version or you're loading it into a daw?
How are you connecting your guitar to your computer?
What drivers are you using?

5. to plug the computer into a mixer you'll just need an audio interface.
If you want to plug it into a guitar cab you'll need an audio interface + a power amp.
Name's Luca.

Quote by OliOsbourne
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Clue: amplifiers amplify so don't turn it on if you need quiet.
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#3
I've played my guitar through my computer. The method I used was really quite terrible and I don't care to admit to it. The biggest difference I found between a real amp and PC modelling though was all in the response. I found an amp brought playing dynamics out a lot more while the Pc seemed very compressed and sterile.

The PC did give off some decent and usable tones though. I just found they covered up a lot of mistakes. Again: this is possibly due to the ghetto method I used of plugging my guitar into the computer. I didn't use a DI box. I believe computer software is a decent way to practice guitar, but I do not think it is an appropriate replacement for an amplifier.