To some extent, it’s all relative… There isn’t much difference in turning-up the bass and turning everything else down.
Except, there’s a digital limit. MP3s themselves and Audacity don’t have a limit, but your digital-to-analog converter does.
Most commercial recordings are already normalized (maximized) for 0dB peaks. So if you boost the bass, most-likely you’ll also have to reduce the overall level to prevent distortion.
You can get the same distortion problem during playback if, for example, you crank-up the equalizer sliders in Winamp, or if you boost the digital signal with Winamp’s “preamp”.
It’s good practice to run Amplify (before saving) to bring the peaks down to 0dB, after any effect that might boost the volume. That will prevent distortion.
By default, Audacity will show red where there is clipping (wave peaks that go over 0dB, and then potentially get distorted when you save or play).
The (lossy) MP3 encoding process tends to boost some peaks and reduce other peaks. So, an MP3 will often show clipping before you do anything. Usually that slight clipping is not audible, but it’s best to normalize before re-saving (which will slightly reduce the volume).
If you boost the deep bass, you will stress your playback system which can also lead to distortion (unless you have a couple of 18-inch subwoofers & big amplifiers, etc.). For example, if you 100W amplifier is cranking-out 100 Watts of 30-40Hz bass that you can barely hear, that doesn’t leave any headroom (extra watts) for the other sound you can hear. So, try boosting the bass at around 100-150Hz, and maybe cut the bass below 50Hz. You’ll just have to experiment to see how it sounds on the gym system.
Try not to over-do it. Try to keep the bass boost to +6dB or less. And when you are boosting the bass, use a known good “dance” recording as a reference. It’s easy to go overboard… Pros often use a reference recording. If you are not getting enough bass from your known good reference recordings, you need to boost the bass on your playback system, instead of messing with the digital file.
One more thing… MP3 is lossy compression. That means you (theoretically) loose quality every time you make an MP3. Audacity (like all “regular” audio editors) has to de-compress the MP3 before editing. When you re-save (export) to MP3, you are going through a 2nd lossy MP3 compression step. For minimal additional “damage”, use a high btrate (or a high “quality” VBR setting) when you re-create the MP3. (Higher bitrates result in bigger files.)
…It’s a different topic but when you change tempo without changing pitch, that’s a mathematically complex FFT operation, and you can get artifacts.