As an experiment, send one tune out to be mastered. Yes, it may cost you a little, but you may learn a lot.
Too much bass is typically a sign of a monitoring environment that is essentially, not flat. That would include bass traps, understanding the standing waves (aka frequency cancellation) and other things like that. I doubt seriously that the board is adding that much bass. Admittedly, they all have a color, but if it's pronounced, something else is the issue. (btw, I too, have those Bx8's as reference).
Try this: Turn down the mix until something is the last thing heard. Then turn it back up some - say 3-4 db. What is missing from the mix? Chances are, whatever the last thing you heard at the lowest volume is too hot in the mix. Essentially, it should sound as 'balanced' playing back at conversation level as concert level.
Also, in your 'car', is the EQ flat during playback, no subwoofer? windows up or down? driving down the highway where road noise is 75db? All things that will effect what you hear.
i wouldn't try and compare you mix to a tune off the radio. Lets' assume you get your mix to sound like that. If you're lucky enough to get airplay, by the time they get through compressing, eq'ing, altering and doing whatever they do to it on the way to the transmission amplifer, well, you won't like it. (I didn't - I was fortunate to have a friend that owned a dozen stations and could sneak it on the air at 3 in the morning - thanks Dave wherever you are now)
I don't know what type of music you are doing, but generally speaking most people like a firm, punchy bass... and so we tend to make sure that 'our' mixes have that in it. Trouble is, most (nearly 100%) people add some form of EQ on top of all you hard work.
I have found that to sound 'reasonable' on other playback devices, i have to leave the bass a little less pronounced in my environment (that's a concrete walled basment for the record). It was (and still is) a hard thing to do.
Note: I'm certainly not saying I'm any good at it have it down pat - I only do this for a hobby)
You metioned a sound proof room, that doesn't automatcally equal an acoustically flat room. Check out
http://www.johnlsayers.com/ for superb room design plans and advice. Also, Ethan Winer
http://ethanwiner.com/ has some good info on acoustics.
As other might chime in with, don't master thru phones, give yourself plenty of breaks - even over days (trust me, it WILL sound different the next day if you've been at it for 4-6 hours the night before).
Also, this is a link to some mastering articles from other sonar users:
http://forum.cakewalk.com/tm.asp?m=432991&mpage=1&key=%F1%A9%AD%9F Bottom line for me is: Try getting an 'unmastered' mix to sound good - that doesn't mean loud - but good and balanced, then master it. The mastering step should just be the polish.
Good luck. Great sounding mixes are a journey. Take your time and know there will be many stesp taken.
(once again, still on my journey)