Hi!
I have one of these from the mid-90's. It is the second one I've owned. Both were purchased used. Mine had the 'HG' designation, which I believe refers to 'high-gloss.'
They are incredibly, exceptional guitars... If you are looking for a spruce top, mahogany body, with a wonderful sound, give it some serious consideration.
I talked with a gentleman who runs a small repair & sales shop and he's been a Guild dealer since around 1980. He told me that I basically had a D-40. There may have been some subtle changes that he couldn't remember, but as he described it, at that time Guild was trying to justify their naming system, or something to that effect.
And I believe the D-40 may have been out of production for that particular period. Most of this is from memory, so I can't say for sure that it is 100% true.
But I can tell you that I have owned several D-25's & D-4's from that same era, and while those did receive their fair share of praise, the kind of compliments I got from the DV-6 were very glorious.
And they tended to be from those whose opinion I admired most. I can only speak mostly from the two I'd owned, because I've only ever played one other in a store, and it was sort of unremarkable.
While I have played plenty of the arched back D-25's & D-4's. The pure quality of tone on my DV-6's are the best of the (or my) bunch. I believe they made the D-6 at some point as well, which as I understand it, is rather similar in specs.
Good luck!
P.S.) I've never played a D-40, so I can't compare there.
P.P.S.) But don't get me wrong, I love the D-25/D-4. My playing style right now lends itself more toward a DV-6. In my experience, you really need to just evaluate each instrument on a case by case basis.