I found this article and I think it address your question:
Here is the link if you want to read the full article: http://www.hifinews.co.uk/news/article/jbl-studio-580-pound;1300/9638
"A glance at the lab report will do nothing to raise expectations of the Studio 580’s sound quality but actually it’s capable of a very appealing brand of music-making – one that isn’t characterised by the ‘me, me, me’, getyou- noticed tonal balance that has characterised some JBLs of the past.
That said, you have to set them up correctly. The key issue – as with all speakers having a rising treble response that you aim to tame by listening somewhat off-axis – is getting the toe-in right. There’s always a concern with controlled directivity horns that this won’t curtail treble energy as required, but you can adjust the 580’s perceived tonal balance benefi cially by toeing it such that the axes of the stereo pair cross behind the listening position. In practice the adjustment is quite critical – I settled for just being able to see the inner tapering side panels. Of course, you could also over-rotate the speakers so that their axes crossed in front of the listening position – an arrangement I like less because it widens the spectral disparity between the direct sound and fi rst side-wall refl ections, rather than reducing it.
In this way you can achieve a fairly neutral tonal balance from the 580 but that’s not to say that the horn tweeter sounds like a state-of-the-art direct radiating alternative. Indeed, would JBL want it to? Play pink noise through the 580 and it’s obvious that it has a distinct treble ‘character’ – one that you must accept if you’re to be happy bedfellows. Me, I came to terms with it and enjoyed the benefi ts the 580 offers. One of which is tuneful bass.
Although the 580’s low frequency output isn’t deeply extended, the nearfi eld measurements show that the roll-off begins quite gradually at about 90Hz and doesn’t achieve the full 24dB per octave roll-off of a refl ex-loaded design until below 30Hz, so clearly the bass alignment was chosen with an emphasis on good transient response.
It’s a design decision that pays off, as I discovered when playing the HDtracks 24/96 download of Christy Baron’s ‘Got To Get You Into My Life’. I’d chosen this track principally for its percussion entry and clean, clear, sibilant vocal, to help decide the optimum toe-in – but I couldn’t help but notice that the strong bass line was melodic and hangover-free.
As it also was on ‘Double Trouble’, ripped from Eric Clapton’s Just One Night [Polydor 531 827-2]. This slightly raw but thoroughly compelling live rock recording is exactly the type of source material you’d expect the 580 to thrive on, and it does. From the moment the Budokan Theatre audience starts to whistle and cry, you sense that the 580 is in its element. Then it’s ‘One, Two, Three’ and the band’s entry confirms that this JBL is both well able to convey the frisson of live performance and, as claimed, very happy to play loud."
I know this article pertains to the 580's but I'm sure it applies to all there horn loaded Studio series. BTW, the crossover for my 530's are set at 1.5khz thus playing some midrage frequencies. Jbl's theory as stated by the article "Horn loading used often to be regarded (and still is) merely as a means to enhance a speaker’s sensitivity. But when you combine a horn-loaded tweeter with direct-radiating bass-mid units, as JBL does here, there’s only so much of this potential you can exploit because the latter will inevitably determine the speaker’s overall sensitivity. So JBL deploys horn loading to different effect: to reduce distortion and increase dynamic range, and to control directivity. As JBL puts it in the Studio 580 product leafl et, ‘Controlled directivity at ear level of seated listeners minimizes unwanted HF interaction with walls and furniture’. Of course, this flies in the face of an alternative wisdom, espoused by other speaker manufacturers, that treble dispersion needs to be enhanced for more realistic sound. But that argument is one that has been raging for decades and isn’t about to be resolved to everyone’s satisfaction."