Should I buy used Thiel CS5

Other way around to me, the tweeter doesn’t need taming, it’s oil can resonance is well above 25khz and low in amplitude
The dac/pre/amp has to be good, so the tweeter doesn’t show up their HF deficiencies.

There is the possibility though that the tweeter if "ferro-fluid" cooled, this will go hard with age and cause non linear sound distortion issues.
B&W need their ferro-fluid replaced around this same age https://www.speakerrepairshop.nl/en/instructions/replace-ferrofluid-in-tweeter/c-36

Cheers George

@georgehifi

Hi I just bought a new (Manufactured 2019) pair of speakers that uses Ferrofluid in the tweeter. How long does it usually take for the tweeter to go bad? Any factors that can make the tweeter go bad prematurely, likely hitting it with abusive loud volumes? I usually play low-med volume...just sayin Thanks in advance.

From what I’ve seen and read 10-15 years is when it can start, depending on environment (dry/humid/hot/cold), how they were used (sedately or punished) and quality of the Ferro-Fluid (dries out quick or slow). People think their tweeter needs replacing when all it needs is a little time and new fluid
  https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1312.R1.TR0.TRC0.A0.H0.TRS1&_nkw=speaker+Ferro+fluid&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=speaker+Ferro+fluid

Cheers George