6-7" for a 3-way?

Have you already fixed on 30L?
To so, can you squeeze in an 8 or 10”

If you must have a 6.5” to 7”, and live in England, then the best 6.5” mid-bass is by Purifi.
The 7” Illuminator also has plenty of excursion.
Why? (No Need to answer)
( Still you're not aware of the mid and tweeter)
Moreover, the price for a driver optimized for playing effortlessly till the mid-hi frequencies and with hi Xmax should be higher than a...non optimized speaker (!!)
That said, I'd choose something like a SB Acoustics SB17NAC35-8 .
I want to try the PAC version! Or maybe not! I fear of the basket torsioning...
Hard to say without knowing other relevant speaker parameters. Is this active or passive? If the latter efficiency is likely to matter. Is 4 or 8 ohms of relevance? Do you want a sub for a deep extension or a woofer for a cleaner efficiency. Ported or sealed 30l cabinet? Does it need to be sensibly priced or are things like the Purifi of relevance?
I'm planning to do active xovers (but might like to try a passive one later if possible).

With an xover of 600hz I don't feel like Purifi or Illuminator drivers are going to have an opportunity to shine, since they excell at low distortion midrange. But I'm happy to have the suggestions!

What is the SB acoustics PAC? I think the ceramic drivers do look nice, but some of Erin's Audio Corner data suggests they don't do so good at higher excursion.

Are there any 'subwoofer' style drovers that can go clean to 600hz?
1. though there are some 6-7inch drivers that model well in 30l enclosure, they usually do not sound well. It is tempting to use it in larger volume, tuned low based on the graphs from alignemnt modelling SW. Slightly smaller volume ~26l is usually better for firm and tight bass. Typical representatives here are 18W revelator, 18WU, Satori MW19, ...
2. 30l is the volume suitable for 8" drivers and if you do not want to spent money for Revelator or Purifi league drivers, RS225 is likely the best choice.
3. SB17 drivers for bass duty in 3way...no no no. They do not do good bass and lower midrange, they sound like with handbrake on all the time. It is better to skip this option. As a midrange quite ok. I know this opinion is not popular here, but is based on multiple experiences in my projects and also listening of commercial product with those drivers + other diy projects. It makes impression of good bass, it goes quite low, but after few songs you realize it sounds still the same in the bass and low midrange, you will miss energy and effortlessness and definition.

Good sounding drivers:
WF182BD09 and 10 ~18l
WF223BDxx ~30l
18W Revelator ~25l
18WU Illuminator ~25l
18W/8545K00 Classic ~18l
22W/4851 Revelator ~30l
NE225W ~30l
Sica 6.5H 1.5C
Satori mw16p, mw19p, though both much better just as midrange

I have hands on, mic on, ears on experience with all of them.
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Ok now that you’ve eliminated 6.5”-7” and you’re doing active/DSP; we can really start talking.

Look at 10-12” and consider Linkwitz Transform.

1. I don’t think a single 8” is enough bass to warrant a 3-way.

2. If I was using a single 8”, I would use a waveguide to help the tweeter meet the 8” in a 2 way.
Ah ah! It must be the worlds Timeline that I'm After you, TK ran...
So OP has ended in going bigger
And my suggestioni was not welcomed or misunderstood (and now I know that the driver must sound bad)
The assertion " good extension - good bass" included the whole design of a speaker: the kind of membrane like aluminum and ceramic.
Well, the diameter now has gone bigger
Nice to now