Ha! That's the one configuration I don't have for my M1s! I have vertical-on-stands, and vertical and horizontal on the wall brackets, but not horizontal on the stand!
I'll do a measure up of the vertical-on-stands ones and give you my best estimate Dave.
EDIT: I reckon you're looking at around 6 - 6 1/2" Dave.
A/V: Sony KDL-V40A12U & BDP-S350, BT Vision, Onkyo 875, B&W MT-30 7.1, QED A/V cables, R.A power cables
Hi-Fi: Arcam A85 & CD192, B&W 602S3 (& the PV1), Partington stands, Grado SR125, Kimber I/Cs, Rothwells, QED speaker cables, R.A power cables.