Hi Lin,
The way I set my system up is to use a Garmin Nuvi 660 and self powered computer speakers. Both the Garmin and self powered computer speakers use cigarette lighter adapters to power them. So no need for an additional amp for power.
You mentioned not enough bass with your current speakers.
I must have tried 8 different computer speakers in my Healey. The ones that fit and sounded the best were:
Sony SRS Z510
Creative Gigaworks T20
The Sony speakers will fit way up in the back of the dash, above the heater box rail. On both the driver and passenger sides. So they are really hard to see, even if you bend down and look under the dash.
The Creative speakers were a little bit larger, so they could not fit up there. They could be screwed into the heater box rail for mounting.
The Creative speakers were the best of all the speakers I tried. Much cleaner, louder and have bass and treble control. As far as bass output, once hard wired and the job was done...Great. I have a lot of bass. Tight bass, not boomy. Nice to have treble control, for I ended up turning that down a bit.
If your looking to hide the speakers more than what I have done with the Creative, then consider using the Sony SRS Z510 speakers. Here’s a link:
https://www.amazon.com/Sony-SRS-Z510-Personal-Active-Speakers/dp/B000095SAX
If you interested in the Creative Gigaworks T20 speakers, here’s a link:
https://www.creative.com/products/product.asp?category=4&subcategory=28&product=14299
Here’s a link to some photos of my installation:
https://pg.photos.yahoo.com/ph/rfc_2002/a...m/ph//my_photos
Hope this answers your questions.
Cheers, Roger