The RD 1750 and the SAZ-1500D are similar/same inside. The design of the Aq2200D is the same, they just added a few more power supply and output mosfets to make it produce more power, but it works the exact same way and uses the exact same parts.
Only some differences are possible between these boards, maybe the components used in the input circuit, and the quantity or quality or both of the capacitors used in the amp, but in real life that stuff gives the manufacturer something to claim that his product is better is some way than a similar product, but really doesn't make that much difference in the sound quality or performance of the amp. So to say that any one of these three amps sounded bad was probably due to some problem with the install or the listeners ears, not the amp itself, as they are so similar that the only difference would be power output in the AQ2200 which would exceed the RD1750 or the SAZ-1500D, but would be equal or very similar to the SAZ-2000D. No SQ differences should be notable, if they are in fact producing different sounds, it should not be detectable by the human ear.
