I've done the same with the dual center speakers and a custom bracket. Be careful of speaker depth and the dash metal. When I first installed mine I had a terminal touching a piece of the dash frame and grounding out the speaker, drove me nuts until I found out why.
As far as bass, I've never found the need for a sub. I always have run a headunit, amp and component speakers + 2 ways and found the sound clear and very full. Having a good headunit with a wide range of adjustments can really help you dial in the settings between the front and rear in terms of bass and treble.