radatats has a good point about "before you spend your money try the L2T or a pair of powered monitors with the HD500". I do beieve running the HD500 into a good pair of powered monitors via the XLR's is a good way to go. I seem to remember many guitar players saying that the HD 500 really sounds great that way.
I wanted some tube generated sound thats why I got the DT50 Head and Cabinet, I never had a tube amp before and I really like this one but I don't haul this beast around to gig after gig. I really think if I was going to play live I would try to go with the HD500 and run direct to the house just so all I had to haul was the HD500 and guitars. I realize there might be problems with it but isn't there problems with anything?
Also, geppert with the DT25 stuff has the right idea ('cause he has experience) about uses of smaller cabinets as compared to hauling around a 4x12. Using the smaller cabinets gives versatility in that you can position the individual speakers in a room in such a way as to achieve different sound direction, not sure if I worded that right but I think you know what I mean.