If you don-t want to spend too much money, you can install a single pioneer / sony power amplifier, subwoofer. Is super simple (rear speakers connected to the power amp, power amp in subwoofer mode, subwoofer connected to the amp. This can be done in 10 min.

the speakers don-t use any fiber cables. is just analog positive and negative cables, regular ones.

you will spend no more than 300 usd for a descent sub. you will not be able to control the subwoofer from the controls of your volvo, is the only issue.

regards

so basically, all owners of 05+ Volvos have to purchase OEM audio? This sucks!
I just bought a RF sub and an amp also! What are my options to bypass this??

The audio in the Volvo use optical cable / connectors. You have two options:

option 1

buy the Volvo Audio package (700usd). This include the power supply, the optical cables, the power amplifier and the subwoofer. Installation included. The pros are the good sound, quality, digital integration, and the control from your console in the car. The bad thing is the price and you lose space in the trunk.

option 2

install an audio power amplifier with a subwoofer and get the audio from the speakers in the car (analog)

Regards

i installed an amp by myself with a RCA hi lo adapter. it look like 3 hours but was worth it. i added two 6x9's to the rear behind the headrests. i cut a hole and put the speakers there. it works fine. i had the base model speakers and it wasnt loud enough. now it sounds GREAT. still thinking of adding a sub. but you can add a sub by doing what i did. basically add an amplifier. its very easy. the 900 dollar system is a rip off. for an 8 inch sub its not worth it. if you know what you're doing you add even add a switch yourself to turn off the sub.. i guess its just about putting in the time. it is ALOT easier to just go to volvo and pay their ridiculous price

I have the '05 AWD T5, so the trunk is already pretty tight because of the raised floor for the AWD. It was mentioned that a new flooring set-up is needed to add the sub, but does that end up making the trunk space even smaller!?

You need to check with the dealer because I believe you don-t loose more space if you install the original subwoofer. The AWD version already have some parts in the trunk, this is why you don-t have space..... you only will need to add the fiber optics, the power supply and the power amplifier..... from volvo...

you will have the same space and you will even see the subwoofer itself (the space is already there)....