Go Active Crossover or Upgrade existing XOs?

I sincerely thank you all for the many varied approaches and results you
have found traveling this road.. This was exactly what I hoped to learn. I am not technologically gifted and have zero engineering knowledge.
I do recognize the wisdom of Clint Eastwood's immortal
words, "A man's got to know his limitations". From what I gleem great rewards await those who spend their lives twiddling with knobs in
the Active XO world.  I will be better off researching the best caps
etc and letting my qualified friends install them. Sad but true. 
FYI: The party suggesting the active route was none other than NP
himself. 

I looked at pictures of the Tannoy FSMs. Hard to tell from pictures, but it appears to be a fairly complex passive 4-way design. On the back I see what look like 4 pairs of R/L speaker cable binding posts.

I have used a high-quality 2-way electronic crossover by Marchand (XM66) with my passive ATC SCM12 Pro monitors. I use the XM66 to split the raw line level signal into low-pass (<70Hz), going to a powered subwoofer; and high-pass (>70 Hz), going to a Wyred4Sound ST-500 stereo class D amp (w/jumpers connecting the low & high pair of R/L speaker binding posts.

The sub I’m using is a very capable, musical-sounding one for my small home office JL e110, and the ATCs are a great combination of highly resolving & musical. Through the Marchand crossover, I’m getting the best sound ever in a desktop system. Note that I also heard the ATCs direct (no active crossover & no sub), and that sound was also fantastic. I’m not sure I could tell them apart, honestly.

Marchand makes great products. For the real audio obsessive, you can upgrade opamps. I’m guessing either the 4-way XM44 of XM9 would do exactly what you need. In your case, you’ll need the crossover frequencies (which will dictate the crossover boards he’ll put in your unit).

https://www.marchandelec.com/electronic-crossovers.html

Final note: Marchand active crossovers use either 24 dB/octave slopes (XM9 or XM44); or 48 dB/octave slopes (XM44 only). These may well be different from the internal crossovers in the Tannoys. 24 dB/octave works perfectly w/my ATCs...

It was recently suggested to me that rather than doing a crossover upgrade

I look into an active crossover for my Tannoy FSMs. Anyone experienced enough

to guide me? What advantages does active provide?

Even if you get the crossover points and slopes correct, there are other considerations to take into account, that active crossovers are unable to account for.

Baffle step, compensating for dips and peaks in individual driver responses, compensating for impedance, etc, etc, are a few issues.

Watch this vid by speaker design wiz, Danny Ritchie. He explains the problems.

https://www.youtube.com/watch?v=kvwsIecOqsU

compensating for dips and peaks in individual driver responses,

You can do this with passive (complex process) or with digital xover (easy)

compensating for impedance,

This to my knowledge can be corrected only owith passive

@simonmoon I agree with you, and I have a preference for well designed and engineered passive crossovers, when you go with digital although easier you start adding variables (emi/rfi, ripple noise, fir filters etc) which for sure will alter the sound