I have, on rare occasions, had mixes that translated so well to anything I played them on, that I could just slap a very mild and transparent compressor on the main mix I could swear it was mastered. Thats what I try to aim for everytime now (pre mastering wise)....since I don't really have access to a totally flat listening environment, I play things on as many mediums as I can.
Some older cars have very flat systems in them(like mid 90's toyotas and hondas)...if a mix translates well in there, I know I am getting close. Moder cars seemed to have very hyped "smile eq'ed" setups, but they are valid as well.
I am a firm believer that you can squeeze quite a bit out of mixing, though I have never sent anything off to Bob Ludwig(yet).... One trick I found was getting a hold of some spectrum analyzers (waves makes some very cool ones that can not only analyze frequency and db's but also phase and stereo wideness), play a reference track through them(if your song sounds like Sonic Youth, load a sonic youth song into your daw and through the analyzer)...observe the frequency response...is it flat...does it slope down at the top end...does it have a huge hump around 80hz? How wide is the stereo field vs. how wide it actually sounds (the 2 differ more often than I thought). Does the db jump up and down or is it totally smashed?.
They always say "trust your ears"...but in the day of home daws...you can't always do that {due to less than perfect listening environments }. I guess with tape, you had nothing to really look at (minus vu's) so you had no choice. Now, with cpu screens, you can see what you are not hearing (what the room isn't letting you hear).