As eluded to above, you need to find ways to get the low end of your mix into higher frequencies. If the kick is "gone" on a small speaker, duplicate (or, better, get close to simulating it with a tom or something that also has some higher frequencies). On a full spectrum speaker, you won't really hear it, but when it's all you have, you hear it. Make sure there's a high-hat or something keeping a beat. Whatever. A lot of making it work is in the arrangement, not the mix. I mix a lot with headphones (I know, different topic). I use a plug-in from Toneboosters called Isone. It makes the headphones have a more realistic stereo image, but also has nice presets for all kinds of different speaker systems, so it's easy to switch from flat to telephone and several in between. I have a 4-out interface, and put Isone on outputs 3/4 and listen to that. That way, outputs 1/2 (master) are always flat, and I don't have to worry about forgetting to turn Isone off