New to me - a very nice 1993 R100GSPD, can't believe I own it...

The Metzler Enduro tires on the bike are old and worn (date from 2014), so the previous owner threw in a new set of Metzler Tourance. Never mounted, but they are from 2015. (dates are 3615 and 1215).

My question: literally riding home on the first ride I notice a bad buzzing through the seat. Nothing in the bars, just transmitted through the saddle. It was low 40 degrees, so on the cool side. The bike handled very well, but the buzzing over 50-60 mph was unpleasant. Previous owner said tires was the issue. Since they are older and they are the more aggressive Enduro tread pattern. But the replacements are from 2015 and I worry they too might be hard. In their favor is the Tourance design (less blocky) and I imagine they were stored inside out of the sunlight. question: Sell them and buy a fresh set, or put them on and see?

Also, it's strange to me, but the 21 inch fronts are bias ply and the 17 inch rears are radial. Didn't think you could mix those.
thanks!