Congrats on your good fortune in finding a pair.
I've been using my original TC-50s since 1985. I drive mine with an NAD C350, which is an integrated amplifier nominally 60/60 RMS and stable into 2 Ohms impedance, with a robust wide dynamic range power supply . I can play louder than is safe (long term) for my ears, at 6 feet. I would not use them for loud rock, given their bandwidth and power limitations, but I can play my electric guitar through a Champ 110 along with the Stones or Marty Stuart while listening to them ;-). I have also used a Quad 303/33 [45/45 RMS] for quite adequate volume in a large bedroom.
As a former user, the OP knows this, but for others: Be aware that at least the originals have a slightly rolled off top end compared to, say QUAD ESL, Stax or Etymotic 'phones. I find them extremely enjoyable - smooth, non-fatiguing, with great imaging (their forte). But i would not use them for critical mixing. I hear upper detail on the above 'speakers' that I don't hear on the SPICAs. They are limited in low bass output (which I don't care about). They are a bit picky about vertical orientation, needing the tweeter to be at ear height, since the tweeters have a thick felt pad around them as a kind of wave guide. They do best away from all walls by several feet. The instruction sheets go into this in detail.
This URL has data, and links to multiple reviews, as well as a history of modifications.
The Stereophile review has multiple authors, and objective measurements as well as subjective impressions.
https://spicaspeakers.com/specifications/spica-tc-50-datasheet.php"Maximum power handling: 50W continuous, 100W peak" per data sheet.
I've never driven mine into audible distortion, since there are no original replacement drivers available. The mfg paid particular attention to pair matching of original drivers and performance.
I have never heard the 'improved' drivers. AFAIK replacement drivers are no longer available for those either, although there is a Yahoo user group that may discuss alternatives.
They are certainly a classic small speaker for those who like naturalistic reproduction and especially enjoy good imaging. Extremely 'musical' for hours of fatigue free listening.
have fun with yours
T.