Recondition my 1986 Thiel CS1.0 or buy new?

Jerryboy66, Thiel might have used drivers that appeared to off the shelf items, but they were in fact modified to his specifications.

That seems like the best course of action. Let us know how it goes.

Elevick, while perhaps not as sensitive as some of Thiels newer models, I think the the impedance load of the Thiel CS 1 is lot easier on amplifiers than the newer Thiels.
Raya, if they're not broken, I'd leave them alone. Though perhaps undervalued on the used market, they're probably not worth $400 on the used market, so the repairs/ shipping appears to be a poor value. They're still good speakers, and I'd caution one against assuming some newer speakers sounding different as actually being better. As much as you might trust your dealer, at this price point, and considering the value of the dollar today as compared to your 20+ year old then costing $1000 Thiels, I don't think the technology has improved that much. I'd imagine you'd be getting a lot less for your 2011 $400 with a new speaker.

Do you mind if I ask what the cost of a pair of Monitor Audio RX6's is?

Thanks. It did end up costing more than 3 X the $400 repair to upgrade. I can certainly understand the extended bass being an improvement. The CS 1's are bit light down there and it draws more attention to the tipped up nature of those speakers. I hope you get much enjoyment from your new speakers!