SPL when designing

I doubt it.
Just have the preamp feed both, and set the Hypex gain to be complimentary.

It might need more “loudness” when the preamp is down low.

Is the sub sealed?
You probably want to pay attention to group delay more than SPL (IMO)

Thanks, @holmz .  Yep, I'm leaning towards sealed.  About 40 liters.

There are multiple problems possible here. Driver as well as amplifier gain mismatches, not to mention the difficulty in finding drivers you really like which happen to match in output, as well as equalization.  Plus, whatever you do on paper you'll probably want to alter somewhat. 

Amplifiers do not all have the same gain. There’s a convention for 23 dBV but it’s not a legal requirement and experimental boutique 6W amps may not follow that at all.

In your case, you may be best served by a very efficient tweeter, which maximizes the output from the 6W amp, and appropriate level controls before the amps.

Thanks, @erik_squires.

OP:

Two more bits of advice.  Consider using a miniDSP digital crossover to set your levels and crossovers, at least to start with.  It will be inexpensive and highly flexible.

Next, take your speaker design questions over to DIYaudio were the speaker makers hang out.

@jgoldrick which model Hypex?
FA25?

On @erik_squires point…

There is a built in DSP on the FA model Hypex amps, but I do not recall if they do a pass through, and/or DSP on that pass through.
Are you running the tweeter with a simple capacitor as a passive crossover? 

@holmz The FA251.  At this point I have a single driver design using the LB8 MKII here.  I plan on an active crossover using this. I made one recently for my main system and really like it.  

I do intend to do a mockup of the single driver enclosure first.  If I don't like it, I'm thinking can add a tweeter/super tweeter to the design.  

One step at a time.

I built a subwoofer last spring using the FA251.  I liked the amp so much I bought another amp and replaced the amp in my B&W 610 subwoofer.

This new build is for the living room/kitchen area.  Probably not more than 75 - 80db most of the time.

Like @erik_squires said, there are many possible issues, but if you have a DSP there will be a lot less!

Let us know what you do.

@curiousjim Ha!  That could be a good long while. I will try to remember

You know that they make an FA252 and FA253 ?

@holmz Yep.  Thanks.