Hi Cyril, sorry I missed your post from a couple of days ago! I think that Joe has a pair of D80R's, and not D38's. I used to run D38's. I never took an impedance plot, but the nominal impedance was 4 Ohms. Which means they probably dip below 4 Ohm at some point. I am sure Joe's D80R's would be the same.
It took a while for me to sell my D38's, and I still had them when I bought my CAD-211AE's. I can tell you that with 110W of triode power behind them, they really got up and sang. No more dynamic compression, no more harsh top end when the volume is turned up.
My suggestion that Joe try bi-amping his speaker should cost him next to nothing, given that he already owns the amps and only needs to buy the cables
It might be worth trying it to see if clipping is the cause of the sound that he described.