I was using a plugin/service to bass boost a song when this question came to mind:
Can you Bass Boost a song that has already been Bass Boosted after being Bass Boosted? In other words, can you essentially just repetitively bass boost a song? If you can, then will it sound good?
Yes, you can feed back processed audio into the same system that processed it originally, again and again. However, whenever you feed back audio into a non-idempotent system, the audio will always degrade over time, especially over many repetitions. The nature and rate of the degradation will depend on the system you're feeding back into. Whether this degradation "sounds good" will be subjective. See I am Sitting in a Room for an "artistic" example, or Re-uploading a video for a similar technical example using video.
Applying an equalizer twice gives (to a good approximation, at least in case of digital EQs) the same result as applying it only once with the controls cranked twice as far from neutral. I.e. there's not much point in actually doing that, just do it once and use the controls stronger.
Now, many EQs have a limit on how strongly you can boost or cut a band. In the analogue case, there are clear technical reasons for this, but for digital EQs it's generally just an arbitrary parameter constraint which has more to do with the fact that more extreme settings would typically just not make any sense musically. IMO that's not a reason to disable them though. The basic equalizer plugin in Reaper, which is also available gratis as a plugin, lets you set arbitrary values, and sometimes this can actually be useful – if you want to completely change the character of an individual instrument or voice.
Specifically bass boosting however is generally speaking a rather counterproductive thing to do, and certainly not a good idea to apply too strongly on a complete mix. The reason is that bass frequencies need to have a stronger amplitude to achieve a given loudness than mid/treble frequencies, so they're typically already maxed out on the mix to begin with. If you then boost them further, you run out of headroom, i.e. you either need to reduce the overall gain (in which case the procedure is better described as a mid/treble attenuation rather than a bass boost), or you need to apply extra compressing/limiting/clipping, all of which leave certain artifacts. (Which can be a valid artistic choice: pumping excessive bass into a compressor is essentially how the ducking effect works.)
Here then it does actually make a difference whether you use two EQs or one EQ twice as strong: in the presence of other effects. Indeed, if you're determined to boost as much bass as possible out of a record, it can make sense to do it in stages: first boost the bass slightly, then bring back the peak levels with a slow compressor and/or soft clip. Then boost some more, and again bring it back with a compressor and final limiter. This combination will typically leave less obvious artifacts than if you handle the same amount of total bass boost in a single step.
But an even better approach may be to not use a standard EQ at all, but rather a multiband compressor. These allow you to strongly compress the bass, thus achieving a higher average bass volume without either exceeding the master peak level or pumping the rest of the spectrum too much.
Yet there's no free lunch: compression always comes at the cost of less dynamic range, less snappy peaks. The only way to truely get much more bass is to use more capable speakers. If you're limited by the speakers, then it can actually be more a effective strategy to remove low bass frequencies that the speakers can't transmit at all anyway; this way you get more headroom and can then turn up the signal louder and boost the lower mids. The final result can thus sound fatter despite actually having less bass frequencies in it. It probably won't sound better though.
