While I had not heard of the "blacktop" tube amp the HD2i is in the build stage and should be a really cool amp.
The HD2 is pretty much a one off and it looks like Craig has taken a further step in his development. The HD2i uses transformers to balance the single ended input, transformers for he output stage and in a nod to Western Electric designs uses transformers as interstage couplers. I own the HD2 and could not be happier, I have heard most amps in the $2-5k category and would not trade for another. While the amp uses the Zana chassis and tubes (the hd21 will probably use a different chassis) they sound pretty different. We had both at the recent NorCal meet and I felt they were both very strong but appealed to two different crowds. The ZD is on the airier tube warmth side without sounding bloated, slow or rolled off but there is a tube warmth present. The HD2 has significantly more power and it is transformer coupled on the output so it is much more controlled and linear sounding. This is probably a factor of three things: Transformer coupling, 6c33b in the hd2 uses much more of the available power and 12ax7 in the driver tube slot over the 6sl7. While I prefer the HD2 I could see a case for prefering the ZD for some tastes. The HD2 also powers speakers, has a dedicated k1000 slot and is balanced.
The "blacktop" may be current workshop item or the ZD replacement, I doubt Craig would place two amps in the same price category and have them both available. The other amp to look out for from him is his Balanced Solid State amp. This is based on the circuit of the Ret Top but has transformer balanced inputs, this will drive k1000s well as it will have double the availble power for the proto red top. He just got the case for this amp and it will be at CanJam 08 along with the HD2i and any other amps he cooks up between now and then.
I really liked the GSX when I heard it but I really have not spent enough time with the amp to qualify differences bwtween the two. I do not think bwtween a GSX and ZD you could go wrong. They are both excellent amps that will afford you many hours of pleasure. You need to figure out if you want a more airy warmer side presentation or a faster more nuetral presentation.
Maybe HumanFlyz or Gran can provide their thoughts in greater detail on both offerings. They both have significant time with both and could provide the best comparo IMHO.
