Crossover for the 604-8G

It seems that the crossover for the 604-8G is still an universal puzzle.

I have a pair of 604 in a very good shape.
I have built a vented box for them
(30 x 23 x 15 inch and a 12 db/oct passive crossover network,
fc = 1500 Hz,
L = 0,3 mH,
c=10 micro F.
Air inductors, paper condensers.

So far so good ! But it doesn't work as I hoped because it seems to be a missing bass and an agressive treble.
Do I have to introduce a damping for HF.? Series or Parallel LC tuned
somewhere between 2-4 Khz RC network? Both of them?
Could someone please help me and send me a diagram for 604-8G crossover?

604-8G main parameters:

- Two way full range duplex - compr. driver + horn
- Sensitivity 98,5 dB SPL 500 Hz - 3 Khz
- Freq. response 60-20000
- Power handling 75 w
- Impedance 8 ohm
- Crossover ~ 1500 Hz

Thiele's parameters for 604 8G
Xmax = 0,15 inches
RE = 7-8 ohm
VD = 19,2 cub. inches
FS = 25,8 Hz
QTS -= 0,25
QMS = 8,30
QEF = 0,26
VAS = 22,05
VID = 0,33 cub. ft
REF.EFF = 4%

I've appreaciated greatly Mr. Parham's paper on "Speaker motors and passive crossover filters".
I've found a link to it in an older message in this forum on the crossover for the 604-8G.
One of the best I have so far read. A lot of thinking ahead.

Thank you very much for your time.

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