First, to correct:
The F3 is a spec of the speaker... you do not set that.
So using your mains, the Dali Spektor 2 as the example, they are rated ±3dB from 54-26,000 Hz, Thus it is safe to call the F3 of your speaker 54Hz.
Half an octave above your F3 on those is 81Hz, and 108Hz is a full octave. For purposes of experimenting with the XO (crossover), I personally, prefer starting an octave above the F3 where possible, and experimenting from there, so I would choose 110Hz to Cross the Spektor 2 at.
Sounds good although some speech can sound hollow at times. Would lowering centre and fronts increase bass?
I don't think so. Consider that the lowest fundamentals of Male Speech is around 170Hz, IIRC. In theory, raising it may help more, as you are allowing the Speaker itself to focus on the upper frequencies, while you send the lower Freq's to the Subwoofer, which is specialized for reproducing that range.
Now, surprisingly, the Vokal is F3 at 47Hz, lower than your Spektor 2s. You could experiment with starting that as low as 90Hz. But again, going a little higher usually helps with dialog clarity, not lower.
I'm not familiar with the Subs from Cambridge. Generally, getting a good Sub will tremendously help your system.
Where are you at? This will significantly impact the options we can recommend to you.
Hope this makes sense!
Cheers!