Heard the B&W N804d3s ...

" Given that the B&Ws have aluminum tweeters, I wanted to compensate for that by a different type of material. Hopefully, it will be fine."

It doesn't really work like that. Unless you somehow over power your main speakers with bass, you should hear little to no change in the highs. If you have an issue with your tweeters, deal with that directly.  

" You should have spend more money on a subwoofer if I were you. I would buy a subwoofer with a ceramic/aluminium coating."

What sub did you have in mind? Not many speakers have drivers like that. I think Monitor Audio and possibly Avalon. 

The old aluminium tweeters of the Nautilus line you own can sound harsh. This is based on the older technique and limited frequency range.

For a subwoofer there are other rules that counts compared to a loadspeaker. In the past I have sold many Rel subwoofers. But to be honest they are rather slow.

In 2015 we compared a few new Rel subwoofers with Monitor Audio subwoofers. These outperformed the Rel subwoofers with ease.

The problem is that Rel spends more money on paid reviews. So people think they are the best. In real they are not the best anymore.

In the last 4 years I sold many Monitor Audio subwoofers which I could use till 120hz because I can set them in phase with S.A.P.- measurement.

When you use a subwoofer who is a lot faster in response you can use it till a much higher frequency. This sets the influence of a subwoofer to a much higher level. This is only possible based on the lighter material they use.

Rel subwoofers can be used till about 80hz, when you would like to use till 120hz, you will hear that they are too slow in response. The control and grip will be gone.

Audio is not based on what you read in paid magazines, but is based on the results you get during shootouts and tests. They give you the real and True information you want and need.

Have you tried a tube amp?

Several years ago I had a Rotel amp driving my speakers. Upgradeitis hit and bought B&W 804S. Big improvement in some ways but now I could hear the limitations of the Rotel.

I auditioned the speakers with some amps and like McIntosh. Final round was with McIntosh MC275 (tubes, 75W) and MC252 (solid state, 250W). I preferred the MC275, which I still have.

Later I added two Rythmiks 12" subs. BTW, they have aluminum cones (not paper) and are servo driven. Great addition.

Later I went active crossover. Now we are talking!! The MC275 drives tweeter and midrange with the passive xo in place. I removed the woofer to midrange passive xo and have Hypex 400W mono blocks driving the woofers directly, crossed at 80Hz to the subs with another active xo.

The heart of the system is a multi way DAC and Acourate software to do digital room correction. might sound like blasphemy to many, but it actually sounds better than ever before.

Cheers!

B&W have quite a variety in sound between models and over the years. I don't think you can simply think of them progressing linearly with minor tweaks but they do produce good quality consistently but often changes are different cosmetics and flavours.

In that sense - stick to what you have!!!!