Reply to some questions I hope not to be totally wrong:
- preamp needed ? Not strictly because for i²s dacs like the Hifiberry, you may use the volume control inside Rune. BUT. It's supposed to be something bad, according to audiophiles. You want volume control to take place in the analog world (i.e. after the DAC).
- To put a potmeter on the thing you need a Pre-amp different than volume control inside RuneAudio.
- With some code writing, and/or a script found on the net, you should be able to open/close a relay with the numerous GPIO on the Pi. so basically, you can use RunUI so switch off the Pi, then during the shutdown process, activate the relay to unpower the Amp board, pre amp, or whatever else you have in there, and then make sure that turning the Pi back ON with a switch will also close the relay to power up everything else. (an init script seems to be your best bet).
When I first started, I just bought 1 dollar worth (read: a fistful) of cheap, through-hole resistors from radio shack, and a cheap prototyping board (with copper rings, so I could could practice heating wire and PCB traces at the same time.) I also picked up a small roll of de-soldering braid for a few bucks. Some people like solder suckers, but I find them overkill for the relatively small components I work with. For all of these along with a cheap soldering iron, you can spend as much time as you want soldering them to the board, then unsoldering, and then resoldering. Just takes a little bit of practice, some observation as to what you are doing, and a tiny bit of patience. To date: no cold shoulders (knock on wood) and no burnt components (knocking again.) 
