never heird of Sachs, unless you mean Sachs Fifth Avenue, the famous clothing store in NY!! I think your mechanic meant SPAX, those are made in England, and they are my next shocks to try; I understand they are nice shocks and unfortunately expensive. It is true there's little to be found about SPAX in the internet, but they have a website.
I have tried Boge, Bilstein, Koni, KYB, Monroe, and Gabriel;
The Boge were a dissapointment, didn't last, they sagged in 6 months. (The Jaguar XJ-6 is a very heavy car at the front). The Bilsteins start out stiffer, sometimes 'harsh', then they start leaking, so my opinion is that they are over-priced and over-rated. The Konis are way too stiff for my taste, even in the 'softest' setting, never again. KYB is a "generic" type of shock, starts out stiff, wears out fast, same as a Monroe or Gabriel.
Skip the hype and get the Spax. Maybe your mechanic knows something I don't yet! There is no Rule about using same-brand shocks front and rear.
My '84 XJ-6 came with GIRLING shocks from the factory, which I saved as a souvenir. The GIRLINGS were decent shocks, not too harsh, not too soft, in fact my car still has the original GIRLINGS in the rear, they seem to last forever, maybe because there's 4 of them back there.
Having said all that, it comes down to what kind of ride you like: back-breaking stiffness or floating-in-a-cloud softness. The XJ-6 was not designed for a stiff ride, but for a luxury-performance ride, which means a little of both.