Sean,
> there tends to be too much bass in them when referenced in the Car and other places. <
This is the classic problem, and it's usually caused by one severe null somewhere around 80 to 120 Hz right at the mix position. Other less severe nulls are surely present throughout the low end too.
The solution is bass traps, and these can range from a few bales of fiberglass stacked up in the room corners to high performance commercial treatment. For starters have a look at the Acoustics FAQ, second in the list on my Articles page:
http://www.ethanwiner.com/articles.html It's admittedly a fair amount to read, but all the basics are presented, in plain English with no math or jargon.
--Ethan