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Both will sound the same. Both will be the same volume. Both will be equally safe. Cost effective... You can buy a cheap Jaycar 1 ohm stable amp and do the 2 ohm subs, that's probably the cheapest way to go but it's not a brand name and I don't trust their reliability 100%.I don't mind spending the money on a decent amp, those two examples you gave helped clear things up which one out of those examples will sound the best? will be the loudest? will be the safest? and would be most cost effective? and that's what I will go with
2 Pioneer TS SW251 10" flat subs
1 Pioneer GM D8500M mono
Thats my advice should be able to pick up new with warranty for $900 ish.
Look im happy to run either 2 ohm or 4 ohm subs i dont really care which, it just seems to be alot easier to find 2ohm stable amps that 1ohm stable amps, im just going for the cheaper option and that seems to be 4ohm subs at 2ohm load