There is a slight difference between really old ones and new. Back in the early 90s there was a change from Revision A to Revision B. There are some subtle differences between the two revs, but both should be fine for LoG-type tone.
One thing I like about the Rev A is that the lead drive stages are both on the same tube, V3. On the Rev B, they are split so that both V3 and V4 have one lead drive stage and one reverb stage. I think its better to have lead and reverb on seperate tubes because certain tubes sound better for high gain and others sound better for clean (reverb). The RevA uses a slightly different preamp tube type (12AT7) for the reverb. Most people on this board seem to prefer the reverb on the old RevAs (I can't tell much difference though).