Seeker,
I will try to give you a possible answer to your question. As I know it,the Aquastar series preceded the Goldscan (GS)series and was the foundation upon which the GS series was built. So, basically, you can consider the GS series the next generation because of the additional design that goes into the preamp circuitry as well as the added ground balance feature.
So, in my opinion the GS 5 and the TDI series will come quite close or may even be slightly better than your Aquastar. At least, the potential is there. Now, I will say the handcrafted units personally built by Eric probably will be a little better than the factory assembly made TDI's. I don't expect the difference to be much but it could exist. I will also say that taking a GS 5 apart to fix is a real pain in the butt and isn't something I plan on doing again.
The GS 5 series and the original 200 TDI's were almost identical except for the single tone feature, the external speaker on the TDI, and the fact that Eric used IC sockets, thus making IC changes easier. The later TDI's are microprocessor controlled so reducing the delay isn't an easy task. The Aquastar and the GS 5 allowed the minimal delay to be reduces slightly, which enhanced small gold signals.
One final note and that is, for the GS 5 or the TDI to approach the Aquastar in sensitivity and depth capability, you have to turn off the GB on the GS 5 and/or TDI.
Reg