After all it does not connect with a super tweeter at 5K(87dB at 5K-going down). Maybe X-Over around 2000Hz max. Losing performance after 3.5kHz. If you want power, I can understand that, give a look at SMC2012/N beyma, with extended frequency range; 1-20 kHz (101dB). If you want to introduce some high quality Super tweeter as the beyma or JBL 2405H, you have to think how you get there either with a compression driver (for power) or a cone/dome mid-tweeter (for a silent environment, home or studio).

Now, I checked about APT’s from Eminence and they seem very good with resonance at 2.5kHz. There is one APT50 and APT80 ($34.99)/APT150/APT200 pre-mounted ‘to your choice of the horn lens’ as an example from the site.
http://www.partsexpress.com/webpage.cfm?&DID=7&WebPage_ID=113
Since your ALPHA 8 rolls-off at 3K5 Hz there seems to be a good possibility...
Again, I like the CP21/F from beyma but I would use it only as a super tweeter from 5K or even more from 7K to 20’sK Hz. I guess you are saving money here.

With 2xAlpha 8’s (96dBs) and a ST324 you need more or less 7dB Attenuation for the high frequencies.
With 2xAlpha 8’s (96dBs) and APT50 (104/105dB) you will need an 8 or 9dB Attenuation. That’s my best guess. I bet that if you ask Wayne P. he can do a good ‘Rolls-Royce/Ferrari/Lamborghini’ X-over project that puts everything in place, with the power and dispersion that you need, and if you use the EMINENCE HF Drivers.
Best Regards.

Thanks for your support
I know about Selenium ST324 & ST322 tweeters. Their price is attractive but I think that you get what you pay. I mean that their response curves are not like CP-21F. Selenium ST324 have poor sound above 15kHz and ST323 above 12,5kHz. Their horizontal dispersion is 120 degrees, CP-21F have 140 degrees. Also CP-21F is 25W AES, Selenium TWs are 20RMS. I bought 4x Beyma CP-21F before year ago and their sound is fantastic but they are in 3-way systems with 4x Alpha 8 and 4x 15" Cerwin Vega woofers with crossover between LF and MF about 800hZ. If you want I can send you photos of them
Now I want to make speakers which have flat response from 80Hz-20kHz (THX crossover point is 80Hz) and four 15" Vega speakers I want to cut to 80Hz like subwoofers.
Alpha 8 have 89db/5kHz and have 2 peaks about 2,5 and 3,5kHz - 97db (it's 8db).
Selenium 12PW3 have 12db (min 92db/450hz, max 104db/3,2kHz)difference.
Selenium 10PW3 have 10db (min 91db/300hz, max 101db/3,5kHz)difference.
THX have requirement for 7db for surround speakers.
I have bad opinion about APT:50
Me and a frend of me used 2x Eminence APT:80 with 4x Eminence Alpha 8 in his car and HF was poor till we mount 2x CP-21F with L-pad

What's your opinion dB

Best Regards
Stefan

'...if you are referring to "THX Approved" speakers, I think that any quality speaker is going to play music or home theater just fine...'
[member in a HT forum]

Speakers complete list of THX testing categories (from www.thx.com Product Test):

Axial Frequency Response Analysis
Directional Characteristics
Sensitivity
Impedance
Harmonic Distortion
Low Frequency Cut Off
Phase Angle
Stray Magnetic Flux
Maximum Output Level
Acoustic Noise Level
Polarity