Need one of those DC adapter bricks that plug into the wall. The one I need is 13.5V, so I'm looking on Ebay and of course there's a lot of them, but some say 1 amp, others say 1.2, and others say 850 mA. Does it matter at all?

The mAH rating will matter as it relates to the current draw required by your box to function well with solid power output. I have a Hitachi TRK-8180E (Euro 220v-21w power consumption) that I run with a 12v adapter that puts out 1A (measured 1.5A) consuming 26w power and it makes a world of difference over those with less available amperage. The adapter never strains because of this.

Using an adapter with less amperage than what the box would normally get to function at its best on DC will result in lower volume and earlier distortion (think like weakened batteries) and with lesser brand adapters, the windings in the adapter will burn out faster because of the strain.

reli
go with the 1.2 amps
its good to have the headroom.
plus you can use that adaptor with other//future
13.5v boxes

OK so how do I know how many amps it requires? None of mine specify amperage on the back of the box. Is it the rating of the fuse inside?

Reli, go with the biggest adaptor you can find as far as amps goes. If it's big box they can draw up to 3 amps when pushed hard (loud). If it's a smaller box 1.2 will be fine. The box will draw what ever amps it needs (it varies with loudness, music and function). Kind of like your ac wall outlets in your house, they are all 120 volt 15 amp (usa), but your not drawing 15 amps everytime you plug something in, only the amps the device needs to run properly.

If you want to just get your box working and
use it at normal volume you can use a low power 200mA wall adaptor.

But for higher cranking volume more amps is better.

A 500mA to 1000mA should be ok for high volume
but more current the better.

1 AMP = 1000 mA
1.2 AMP = 1200 mA
850 mA = 0.85 AMP