The original seats in the 300D are fairly firm -- bonded horsehair pad with some thin foam under the TEX. Not to everyones liking, and you must fiddle around with the back cushion to get comfortable. Lack of bottom cushion tilt can be a problem, but you can fix that within limits with some spacers. Helps to keep the seat a bit higher as you have discovered.

One thing I have never found to be true, though, is lack of back support. Check the padding -- if it crunches or has soft spots in it (or circular hard spots), the padding is shot and needs to be replaced. If you've had water leakage, the dampness can deteriorate the padding badly.

The leather seats had cotton padding in the pleats on the 280 -- I've replaced it with 1" polyurethane foam in my homemade covers. Much better seating, since the foam spreads the pressure around a bit. Seats are "damper" since the channels squash flat under my ample sitting portion, but better for longer trips.

The cotton in the 280 was hard as all get out, completely flattened in the 30 years of use.

If the pads seem OK but the seats are still too hard, you can get sheepskin covers. This will soften the seats a bit without major work.

Removing the wallet helps A LOT!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!