8ohm 2W seems to be a safe speaker for all the major boards. Going through my manuals...
NEC boards only specify 8ohm speakers.
1.5-2W for Spark 2
1.5-3W for Spark Color 2 and Igniter 2.
NBv3 says a 4-8ohm speaker, 2W Max.
CF 8 says 4-8ohm speaker. 2W (though no mention of maximum).
SF seems to use either 8ohm speakers or 4ohm-speakers-in-series-with-4ohm-resistors for their ASP Champion kits, so it seems that SaberCore is intended to use an 8ohm speaker.
Beyond that~ Dunno. Audio tech's something I've only just recently started getting into, so I can't offer first-hand experience.
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Vos pretty much covered the kill switch. only thing I'll add is sometimes folks'll wire multiple positive wires or multiple negative wires to the battery leads, depending on board setup. If you've got that going on, either
1) Splice the switch into the lead with the single wire (So if there are two positives and one negative, cut the negative wire and use the switch to bridge) or,
2) (Assuming two positive wires and wanting the switch on the positive line) De-solder the multiple wires, solder one new positive wire to the battery positive, join that wire to the switch (either end or middle), then connect the remaining wires to a switch contact right next to the previously soldered wire.
As long as you're using two switch contacts right next to each other and leaving the third unused, it'll function as a kill switch.
This is that AIO chassis that doesn't have a recharge port, right?