Hi,

the stock amp is a 6 channel amp which produces 6x22 watt.
2x22 watt RMS goes to the front system
4x22 watt RMS goes to the 2 subwoofers, which are double coil speakers.

Double coil means that each sub has two coils at 4 ohm.
You can wire them as one 2 ohm coil.
And by doing that you only need a 4 channel amp that is able to support 2 ohms.

If you plan to to use the stock speakers i wouldn't use an replacement amp that produces not much more than 4x70 RMS watt. Else it's possible that you blow the speakers easily

If you want to change the stock amp, please consider that it has a simple buildt in processor and a crossover network for the subs. So it wouldn't sound better by just replacing the amp – you'll have to find the right settings and i would recommend a head unit with a built in EQ and Crossover for the sub.

If your stock amp is o.k. i would recommend to replace the head unit only with a self adjusting processor head unit and put some tweeters on the dash. We've done that with an Alpine HU and an Alpine PXA-H100. The Tweeters are amplified by the HU amp. Sub and TMT are amplified by the stock amp.

Regards
Markus