Ken,
I've Worked on a bunch of 400s. Some from Sound Sales and some not. In terms of quality, After about #1235 or so, they beefed up the frame. Regardless of the stickers, these machines were all made in England by the same people. Sound sales "improved" the SMS 2 with the SMS 3. To be honest, I can't see too much of a noticeable difference. The relocation of the brackets on the spill box, in my opinion, was a noticeable improvement. It strengthened the frame and added torsion resistance, and it made the spill box location adjustable to a degree so that you could better align it to the tapes. Those silly wooden wedges are cute, but trust me, they come in all shapes and sizes and vary from machine to machine. As for the power supply, it is my understanding that there were early models and later models. Sound sales had little to do with that. I have Sound Sales Machines - (400SM 1562 & 1334) and one that is not (M400S #1463) These are all "late frame" machines. I recently restored 1225 which was an "early frame" machine with the wooden wedges. Once properly dialed in, they all play great. With Sound Sales you get the big tacky decal on the power supply and the back of the cabinet. So what. Also, it appears that from what I have seen, the Sound Sales machines had a single preamp card instead of two. Also, the "late frame" machines had a different keyboard frame as well, and different bracing. I think the later frames made the machines somewhat more road worthy.
I have literally hundreds of photos of the restoration of both early and late frame machines and Sound Sales and not Sound Sales machines. Send me your address and I'll load them on a CD and send them to you. You're welcome to put my work on your web site.
Frank Stickle