OK, my experience with TD's has been new Bilsteins on the fronts (both wagons). I did replace the rears on one after they blew oil on a frigid day in W. Texas (twas a bad trip, that one!)
I had a bad experience (well, not really bad but I was unhappy
) with HD Bilsteins on my 123 300D. I gave them about three months to loosen up but I finally decided to return them (the local place I got them from was happy to accept them back and that made me happy!) They never softened up enough to please me.
My '83 300D had new comforts on the rear whe I decided to try HD on the front, I don't know why I got them in the first place. The front end felt real stiff and driving over little bumps transferred the shock to the driver, then on to the rear end which seemed to bounce, it was a mismatch for sure. Maybe for someone who carried a heavy load and drove mountainous roads would like HD's. The nice smooth yet strong balance Mercedes achieved in the 123 requires comfort shocks. Heavy Duty shocks aren't able to give that balance.
KYB's? I put new KYB's on my W115. They were truly sporty feeling and I liked the "Keep You Bouncing" acronym someone on the Shop Forum came up with, they do have that gas shock feeling to them especially when new. Unfortunately I had to replace them with Bilsteins because they started leaking fluid. I'd see a small puddle of oil on the garage floor right under the shocks, I'd mop it up then it would return. It started with the rears. If you read the papers KYB puts in the cartons, it says something about "small oil leaks are no reason to return these shocks, this is normal and will not blah blah..." I remember that well, after maybe just 2 years of light use my KYB's were leaking to the extent that I figured the oil would be gone sometime soon (how much oil can they have in them in the first place?) so I got rid of them.
Bilsteins are the best! I am not too proud to say I learned this the hard way!
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 296650
1983 300D 243280 been in the body shop for 1.5 months for a new hood! (dont ask!)
1985 300TD 223470
1987 300D 262300 my diesel road blaster! get otta my way! (new Bilstein shocks and struts up front! woweee!)