Just upgraded form the SWR Workingman's 4004 to a 550x head and I thought I’d share my experience. Both heads have that signature SWR sound that I happen to like. As a matter of fact, when I plugged in the 550x I initially couldn’t hear the difference between the two heads. They sounded exactly the same. But as I played, I noticed the 550x was slightly warmer and had slightly more chest-thumping punch. I’m assuming the warmness is from the tube preamp and the punch was from the extra power (550W vs. 400W). I think the WM 4004 with its 5-band eq actually lets you sit in the mix better than the 550x with its 3-band eq. Having separate control for low mids, mids, and high mids allows me to sit in the mix right where I want. The 3-band eq and single mids control on the 550x makes my tone sound a little lost, but this problem is outweighed by the slightly improved qualities mentioned above. I really expected a bigger difference between the Workingman’s and professional series, but tone wise there is not. I guess you’re paying for additional features and power. There is about a $400 difference in the price of these two heads when bought new. I don’t think the upgrade is worth that much money, but then again I didn’t pay $400 to upgrade so I’m keeping the 550x and selling the WM 4004. The WM 4004 is one fantastic, reliable amp.