The heads the Chinese are selling as solvent heads are actually Roland-packaged DX4s with the factory solvent manifolds. The water based DX4s are ripped out of Epson 10000 spare head carriages, which go for a bit less. (It used to be about $750 per carriage here in the US, or about $225 per head, before Epson shut the barn doors...)
For what it's worth about the head ranks, I've done some experimentation using different head rank values in a solvent Toucan LT. It seems that the first few digits of the head rank (6E7, 8A1, etc) indicate the voltage level it runs at: the higher the first three digits, the more output that head seems to have.
In the JV3 printers, the head rank can have a pretty pronounced effect if you try running, for instance, water based inks. I tried to convert a JV3 to SEPIAX inks and immediately found that all the heads were firing two small double drops where they should have been firing one normal drop, because of the change in viscosity of the ink. Using a head rank with a higher voltage setting caused the heads to fire normally again.
I was also recently able to bring four different Toucan LTs back into line just by changing the head ranks on the C,M, LC & LM heads. (The goal was to have all printers printing almost the same grays, without having to run a new linearization curve for each printer).
If you only have one machine, however, the difference is generally so subtle it's difficult to notice and you'll have better luck just re-linearizing your printer.