Hi Retro Soulman,
I'd like to help you but I need to know a few things also.
RE: "They sound a little boomy"
I modeled the driver in Winspeakerz to try to see what may be the cause. Standard 4507 is said by JBL to have 5 cu.ft. (142 L.) tuned at 40 hz.
RE: "I close the 2 vents" As seen on a picture of original 4507 that box has FOUR vents originally, yours has two. You may have made two larger ones equivalent to the four, I don't know. But this raises a question in my mind: are your boxes an EXACT copy of original in terms of volume and tuning frequency or are there some variations made to the original box design ?
Since you indicated the boxes being your studio monitors, their placement is another question. Are these located on the floor away from wall(s) or on a wall or floor/wall junction or near a corner?
In the first picture of modelings done (5 cu.ft., Fb 40 hz), the 45 hz bump in response is at a level about 1.25 db below the 98 db darker black line (i.e. driver rated sensitivity). Then response rises progressively from about 100 hz up. So I'm wondering what are you hearing actually? The 45 hz peak, the rising mid- bass or the combination of the two? And/or some room effect depending on box placement (room gain)?
In the second picture (3 cu.ft./tuned to 50 hz, F3 48 hz) it can be seen that response is flatter, though not flat, which indicates to me the driver is more "at ease" in a smaller volume box, with less droppy low end.
In the third picture you see both modeled response curves kept on the same graph for comparison purposes. Personnaly, I much prefer the response curve of the smaller box since its flatter than larger box but doesn't go as deep...
Note the red (solid and dotted) lines on graphs are woofer excursion (limit is dotted, actual is solid) and the modelings done as usual with 1W, half-space and QL 7.
The 4580 driver isn't made for deep bass output, its more for high level sound without really low bass capability. Some driver parameters point in that direction: Fs 31 hz,
Qts 0.23,
Vas 198 L. (7 cu.ft.),
Xmax 2.3 mm. Qts is small, Vas not large either and Xmax is quite limited. Qts and Vas both influence optimum box size and here they point in the direction of smaller. Xmax says here don't push me too much VLF wise.
BTW for the 4507 box JBL recommends the 2225 driver which has a higher Qts of 0.28 and Xmax of 5 mm.
I tend to agree with you that 4507 may be too large for the driver, though Gauss uses a 5 cu.ft. box as an example in the data sheet. The shape of their response curves is similar to what I get. I tried different combinations with reduced box size using other tuning frequencies.
I stopped reducing box size at 3 cu.ft. NET (85 L.) because smaller than that you may start getting problems fitting the 15" woofer in the box! So, this in my view is more a driver for 3 cu.ft./ 50 hz box than a 4507. Regards,
Richard