The X1550 I had installed on my X5000 was a Sapphire 256MB model with DDR2 ram so am perhaps there is a subtle difference here with the one you have tried..
The X1550 series does appear to be a little hit and miss as when I was doing a little bit of research I came across the following which highlights that for some X1550 cards the U-boot May not appear.
I’d try to find a X1650 instead as the performance for 3D in MorphOS was much better, but the one I was using does not output via DVI in MorphOS.
Any idea how close the development team are to being able to support the 7750 series cards?
author of www.Amigax5000.blog