DSS - Yes, it really works. Dummy loads are used frequently by amp techs, audiophile guys, etc. As for your output trannies, you don't have to worry. Provided that you wire the jack properly, use the right cables, don't feed it more than 100W RMS, and use the proper impedance you are fine. I've used the berry/dummy load on many a head and never had a single issue, and yes I have dimed them.
The 9V battery would work, but you're better off with phantom power. That VTB1 tube mic pre from SP is friggin GREAT - I love mine. $99 bucks with free shipping, fuggiddabodit' Donnie Brasco.
As for it sounding better than the line out on the XXX, yes, absolutely it will. Again, it is not going to be the ultimate/greatest direct sound you have ever heard, but its a very inexpensive piece of kit. You can choose to believe me or not, but I had the Mesa Recto Recording pre for a VERY short while (very disappointed with that thing). In any event, the "direct' outs on that thing, which Mesa said were many years in the making, tons of research, blah blah blah... Well, I shit ya not but it sounded better direct using the speaker outs and the UltraG vs. the factory Mesa emulated outs...
I have not even looked at the HC reviews on it prior to this, and I guess I'm not alone in liking it. Here's the link:
http://www.harmony-central.com/Effects/Data/Behringer/Ultra_G_Direct_Box-1.html