If you still have a stock radio. I would upgrade to an after market if installing an amp. The updated new radio will sound much better + have all the needed extra music + subwoofer connections if needed. I have a small 700 watt- 4 channel amp for the speakers under the passenger seat + my sub woofer amp in the rear hatch area.
I recently up graded my outdated 2004 aftermarket radio in my xB last month, cause i wanted a Aux input for MP3 player etc. + a USB port, which can play music with a memory stick or I-phone/ I-Pod.
Believe it or not i found a great buy last month from Crutchfield Stereo. I only usually use them for only info as to what fits im my car or reviews, since they are kind of expensive + find it cheaper elsewhere.
But they had a good buy + this what i puchased
JVC Arsenal KD-AHD79 with HD radio CD receiver $129 with Three sets of high-powered 5-volt preamp outputs not the usual 2.5 volt, two Aux ports ( rear + front), USB front port etc. Rated on the internet best buy for budget headunits. Warranty: 2 years
And remember you get all this with free shipping, free installation guide, free $15.00 Stock adapters to plug in your new aftermarket headunit and a free $15.00 receiver adapter box to fit your new receiver in place of the OEM radio spot + free excellant technical support if needed while you try to do the install. Just double check the Tech hours.
You can also look a the crutchfield site to see what speakers fit in your car. But, may wish to purchase on another site since they sometimes are more costly @ crutchfield. If need you can buy the aftermarket speaker adapter plugs or the correct aftermarket new speaker adapter @ Crutchfield if needed for the xB.
I would suggest to buy the "Bass blockers 800 Hz" to install on the new aftermarket small dash speakers to avoid blowing them out + protect your speakers from bass distortion and enjoy cleaner sound with Bass Blockers http://www.crutchfield.com/shopsearc..._blockers.html
Crutchfield customer service
: You can make believe you wish to purchase something + they will guide or answer your question = FREE Technical INFO for your install and you can buy elsewhere cheaper. LOL
If you have any further questions, we are here also to help.