Those brands of amps sound pretty good, I'd go with the PPI, 4" are definitely
very small, I'd go with at least 5 and a quarter if I were you, I don't think
you'd be happy with 4". But if the custom installer charges you an arm and a
leg to put in anything bigger, get the 4". Don't get a 2 channel, you'll
regret it, get a 4 channel amp, you can find a cheap decent one, that way if
you replace the polks in the rear down the road, you won't have to buy another
2 channel amp