The reason the REL will improve your whole system is because it is not only about low frequencies. Even mid and high notes encompass a much larger spectrum than the principal note you hear. For example: if it was only about the principal note being played, then you would not be able to tell the difference between a piano, violin, oboe, flute, etc; if they were playing the exact same note. Also why a concert grand piano sounds different then a small upright piano. There is a lot more going on.
The hardest thing for a full range speaker to reproduce with detailed accurately is bass. The REL will fill in the bottom of all notes being played by all the instruments, which will make your midrange fuller and enable you to differentiate between even slight changes in the high notes. If you listen to classical, it will also help separate the low notes played by the cellos and string bass, resulting in a more musical experience.
Since you are willing to spend $1499 which is what the wood veneer Stratus III costs (list price),(black is $1299); you could go for a Storm III in black which lists for only $200 more ($1699) and is a good step up (wood veneer Storm is ($1999) Places like Ambrosia Audio usually discount.