If you get speakers that can replicate the sub sound, with a treated room, and headphones with good bass response (or any other tool to help you feel ie sub pac) you can get a good understanding of where the sub lies.
I spoke with
Crizzly, who happens to be an old high school friend, a few months ago and i asked him about the same thing. He said that he ended up just mixing with out a sub most of the time because of how much of the replication he gets out of it. It's a little over bearing at times, but a good indicator of the feeling.
I also read up on a few articles about how having one sub bass in your room can actually make things worse if you're not careful about how the room is treated. this is due to some phase issues i believe. One way to eliminate this by having two sub woofers on either corner of your room (if square) to eliminate the issues that a a single woofer may cause.
Another tip i read waaaaaaay back when i first started, comes from a sub focus tutorial i think. He said, you can get a good indication of feeling by putting your hand over the woofer of the speaker and just feeling the sub. I use this technique a few times on my yamaha hs8's.
Now, i have my mixes go through the round table of my production circle to see if i missed any thing. typically they don't have any thing to say negative about my mixes, which is good and bad lol. But the point is, you don't really need a sub, and in fact getting one with out proper calibration could just make it more of a pain than what it's about.
