I would definitely look at a modern wave guide before going the diffraction slot horn-way. To me the M2, 4367 and even the el-cheapo VTX-F12 WG does sound better than many expensive horns with a diffraction slot.

It is a surprisingly small difference between the el-cheapo plastic JBL#506815 and M2 WG and in my world smaller than between the new generation WG’s in general compared to the older horns. Maybe that is just me.

Since the #506815 WG is so cheap I would at least give them a try before venture into more exotic (expensive) horns. The fact that is cheap, small, uggly and made in plastic does not change the fact that they sound and measure good.

Regarding 2450SL vs 2451 core I have always thought that they are identical. They are, as far as I can figure out, not. The phase plug is different and sits differently in the 2450SL. After having moved dia’s (Ti, SL, Be) around between them I have also come to the conclusion that they do measure and sound different. Differences are small but given a choice I would opt for 2451 core.

With a proper SL diaphragm (preferably D8R475Nd or D16R2451SL) they measure and sound wonderful on the waveguides mentioned above.

Be's are also fine, but as stated above, do benefit from an UHF unit as in all JBL K2 speakers.

My 25 cents.
//Rob