First of all, there's no difference between 2 x 1200AS1, and 1 1200AS2. I had 2 x 1200AS2 and 2 didn't sound better than 1. Just a waste of money. The $300 module sounds the same as 2 modules for $500.
Secondly, I tried many of the tweaks you mentioned myself. Since the connectors on the module are JST (tin), again, no improvement on the sound quality - even when using direct connect methods. Do you really think using solder instead of the JST connector makes a difference. All these tweaks amount to nothing, other than an excuse to inflate the price with the appearance of added value.
Forget about Manufacture warranty since you've modified the boards. What happens if the changes you've done to the boards causes damage to your clients other equipment (like speakers), are you going to cover the cost for that too?
These modules are so cheap that if the main fuse on the module goes, the manufacture won't even attempt to fix it, they just replace the module.
The only reason you chose these modules is because of their low OEM costs. Why didn't you pick a different module designed for Audiophiles?