What you are hearing could simply be what is really in your audio source file, as you already stated, maybe you haven't heard this before in your existing system(s).
Most consumer speakers, except for high-end “audiophile” systems, are designed to cover up audio artefacts and faults so that the listener hears acceptable, if inaccurate, sound. Both artist releases and commercial releases often have some such issues. The main reason is that the recordings are mixed and mastered to be broadcast, in some cases (such as commercial releases), and to be played back on a huge variety of systems, including all sorts of home systems, computer speakers, and even lower-end car speakers. Therefore, compromises are made to get the most acceptable overall sound on the widest variety of systems. It can be worse with downloads because MP3s are heavily compressed, but we assume customers are not listening to MP3s for a speaker test.Perhaps that's our error. Listening to accurately recorded and rendered audio is important in determining problem areas in speaker design.
In contrast, a studio monitor—the Eris E3.5 BT or Eris E4.5 BT are essentially the same design as our other Eris studio monitors and can be used for home studio work—reveals much more accurately what’s in the file, faults and all. If you haven’t listened to your audio file on accurate studio monitors before, you don’t really know what’s in there.
So if the audio source was significantly compressed during production or mastering, for instance, you would hear what you describe on accurate speakers.
If the noise you hear is a hiss, that could mean that your other system is not accurately reproducing the high frequencies where hiss resides. It is less likely that the large amount of low end in your other system is covering up other frequencies but it's possible. In that case, adding a subwoofer as a solution basically means you are trying to cover up what the audio actually sounds like - make your system less accurate.
Either way, the point is that your other system may not be reproducing the sound accurately, so when you get accurate speakers, you hear what’s really going on in the audio, and you are not accustomed to hearing that with other systems. You’re hearing the highs better with ~ than you usually do. Adding a subwoofer is only a solution if you are trying to obscure the audio by overwhelming everything else with bass. With a more accurate speaker, better recordings will sound their best, and you’ll hear the faults in compromised recordings.
Eris E3.5 BT or Eris E4.5 BT speakers are designed to provide the best possible sound quality at the price point and not compromise the audio source material. To this end, we chose not to use noise-suppression filters in order to take noise out of a bad recording or MP3 file. Such filters would degrade the sound.
Furthermore, we are convinced that the market is moving to uncompressed audio, and the Eris E3.5 BT or Eris E4.5 BT speakers are ready for this.
If the issue persists, please contact PreSonus Technical Support by creating a Support Ticket at My.PreSonus.com/Support for further assistance.