Any of the Volvo amps will be plug-n-play. The later amps will require a separate 6-pin din cable from the head unit to the amp; the early amps had the cable built in. Check the specs in your owner's manual; your head unit is probably putting out 4 channels somewhere in the 20-25 watt per channel max range. Adding an external amp probably ( I don't know for sure) will cut off the internal amp, so there's no gain unless you add an amp with larger capacity.
You'd also need to rearrange, or get appropriate wirng adapters/harnesses for your vehicle wiring, to get power to the amp and have the speakers powered from the amp rather than the head unit. Ebay is not a bad place to get educated on volvo amps, after you have read all the posts in the archives.

All that being said:
1. there are instructions here on the board for adapting the 6-pin din plug to an aftermarket amp input;
2. Your best bang for the buck may be in upgrading your speakers, not your amp, if you still have the factory speakers; and
3. whatever you do, start with the correct wiring diagrams for your car and document all changes/ inserts so that a year later you will accurately remember what and where you made any changes.
Good luck.
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We have met the enemy and they is us. [Pogo] S70 T5 cop car : Rough Rider suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps