an update about my system : now the 4MB SRAM is up and running (actually 64K of it as I only use a Z80 currently).
I modified John's monitor to make it work without the console I/O board, quite easy. I had to force the buffered A20 and A21 address lines of the SRAM board to ground though, having a passive backplane there is not pulldown resistors.
I wonder now what the next step should / could be.... I am quite tempted to build the SMB, I think that would be of great help for forthcoming debugging...
Having lots of fun !!