Thanks for the response folks.
The power point tester is my next purchase...possibly the cheapest and easiest of the process though it could lead to some expensive modification.
Thanks Noel, that's a cheap one that can be had fairly locally.

Attached Files (Click to download or enlarge) (Only available when you are logged in)
no connections.png (7.44 KB, 159 downloads)
Nothing connected to the interface.
bal cable IN.png (11.26 KB, 160 downloads)
Balanced cable (XLR) connect to the interface.
bass to DI GND 20dB attenuation bal to interface.png (32.52 KB, 157 downloads)
Instrument to DI unit. DI unit to interface via XLR cable attenuated.

Cheers
rayc
"What's so funny about peace, love & understanding?" - N.Lowe