After looking at this in some detail, here are some comments:
A FE166NV in 10L will just not get you much under 100Hz. Even if you run it standalone, you will likely need at least a contour notch filter (anti-shout filter) on the 166 because that's where the baffle step will begin to happen on your 10L cabinets, too.
Adding a tweeter in combination is likely to sound unbalanced (not enough lows) because that speaker will not reach very low in the sealed 10L cabinet. If you have subwoofers or something else below ~90Hz, balance will be fine and it's a good approach, IMO. The other alternatives are to (a) just run the 166 alone (w/ anti-shout filter) or (b) use corner/wall placement with a different (reflex or other backhorn) enclosure capable of elevating low-frequencies to restore balance. In that way, the minimum attenuation of the rest of the responses is possible. Otherwise, you can filter the life out of it and it will seem to go lower but you will be attenuating a great deal of the response. It is not possible to comment further w/o knowing where you will put them (relative to walls) and how far away you will listen.
Nothing says you can't do that, it's just that the 166 gets even a bit rougher (~10dB after processing out the IEC baffle and adding in yours & the sealed-box response) so it all depends on how flat you want what or alternatively, how loud. Here's that much--before any filtering at all:
If you add the tweeter, you will have to pad down the high-frequency response as it is more sensitive. If not L-pad, fixed resistors something like 4R series and 8R parallel (-6dB into 8 ohm) should help balance some if you are just playing with loose parts. All that is *assuming* one wants to really go after that peak-trough a little over 1kHz. If you commit to an entire crossover network instead, then the attenuation may be incorporated into the filter.
Instead of 5kHz on both, an asymmetrical crossover of say 4kHz on 166 and 6kHz on tweeter seems to work better for me in sims.
If you choose 2nd Order (or more) on the 166, you may benefit from a zobel (say ~8uF 10R), but will also likely need to reverse polarity on the tweeter.
Questions:
- for close listening or for farther away?
- do you already have the 166NV?
- do you want the tweeter inside the sealed cabinet or free to move sitting on top (& how far apart?)
- any chance you measured your tweeter impedance (or have the data elsewhere)?
- some people very much pursue very minimal filters -- is that your goal, too?
Not sure how much experience you have with fullrange drivers, but it is often found that some off-axis angle is the best compromise for balance while preserving the sound one appreciates in the driver. People seek different things. Some people find most Fostexes shouty, screechy, harsh and annoying. Others find them lively and detailed, etc. For these, the flattest axial response is not always a useful goal as filtering complexity can defeat original coherence often sought. Leaving the speaker "hot" on-axis and then listening off-axis is a common approach. Other people (close/monitor-ey) want the opposite--flat axial and forget the rest. Still others wont touch Fostex/Lowther etc and seek flatter responses from (say) Markaudio, etc.
Without real measurements with the actual driver positions, filter values become guesses--here's one as an example. Using 9in as the width (front), the tweeter 150mm up from 166NV, and splitting pwr and axial response priorities 50/50, this is a quickie filter out of XMachina:
You will notice that considerable output has been reduced in the process of "flattening". If that's objectionable (say low-watt tubes, etc), then another approach will be required. I also have not gone to the trouble to verify that in vcad, etc.
Honestly, anything with a whizzer is a complete crapshoot w/o real measurements--and even then final filters need to be done "by ear", IMO.
If you already have the 166NV--try it by itself with a RLC notch seriesed. If you have them laying around, you could try a parallel combination of 2mH+10ohms+1.8uF in series with (only) the FE166NV as a first guess if you want to experiment with it standalone.
Some people want tweeters just for "air" and point them backwards and cross even higher than 5k and that's good enough to allow listening further off-axis to the main driver and they just leave it screech
All this stuff depends on what you want to do, what's important to you, and how you want to use them (and on what program material). There are any number of approaches & speakers are nothing but tradeoffs, but those are some of the considerations. If, for example, you want max output and a simpler filter yet and just live with it, you could get some output back and reduce parts. Tradeoffs.
