I just got back from my local Eden dealer with a new WT-550 under my arm. I had the chance to A/B it side by side with a WT-400 thru a D210T and a D210XLT. I set the controls identically on both heads and alternately plugged them in and listened. The dealer told me they sounded identical, just with more power in the 550. I said, "If that is so, it won't take but a couple of minutes to find out." In fact, I have had more than one dealer say they sound the same. Well sir, it only took a couple of minutes to find out that they DON'T sound identical. The 400 has a higher treble content than the 550. I had to back off my passive tone control a good bit to lessen the sound of the strings hitting the frets. There was a slightly snarly sound to the strings as compared to the 550. Some might call this growl, but to me it sounded twangy. The 550 by contrast is deeper, clearer, with less hash, and controlled the lowest notes with authority. This control could be the higher power kicking in. I'm not sure if that is the whole story, though. The degree of control and effortless sound was present at all frequencies with the 550. Now, it might be possible to get the 400 to sound like the 550 with the proper EQ. I basically had the controls set flat with just a bit of boost in the low and mid bass. The enhance control was set at 12 O'clock. To this tone junky, there was only one choice: the WT-550!