12v linear power supply

Most DACs or preamps or small a/v devices will use less than 1 amp, so a 2A linear power supply will be just fine.  If you want to go for a 4A, it will not hurt anything. 

If you have something like a video processor device (such as Lumegan) or a computer type device like Roon or Intel Nuc, then you may want a power supply that can provide more amps.  The Keces P8 that provides 8A is the most powerful linear power supply I know of, but it's $900 retail! lol.

The Swagman is at a similar level to the Teradak and Zerozone power supplies.  They are just fine, but not as good as Sbooster.  I think one of the better supplies is the Farad Super 3, but that is also expensive at around $600 or so.

FYI, the ifi audio iPower Elite is not a linear power supply.  It may be better than your normal switching "wal-wart" power adapters, but it is still a switching power supply.

Zero Zone Ss2021. High quality torroidal with a large bank of capacitors, i.e. very stable and safely controlled. I use it to drive my Etherrregen and am very pleased with the performance. Spend what’s left of your $300 on a good fuse and power cord or a sterling silver DC cable. All on AliExpress

Automotive battery should be pretty good for that. Some of them can last before recharging driving your preamp for months.

Get simple low-current charger for it and you're all set 

Little more but Teddy Pardo is an excellent LPS. I have one with my RME-ADIFS and there is a noticeable difference.