X128 is available from World of Spectrum. Here is a link to their emulators for Amiga
I think CBSpeccy will run better than X128 on 030 Amigas... it also emulates 128 models and supports .TAP + .TZX formats, but is a teeny bit unstable iirc.
About the ZXAM speed issue... I think my memory is at fault here. I was probably thinking of Speculator...