The composite output on the A600 is in colour, so if you could use that instead of the A500, you have all you need. Or if composite is not good enough and your big TV has VGA, you could get an Indivision ECS and then connect the VGA output of that to the TV, and the original Amiga RGB to the 1084, and STILL have the composite signal to use when recording
The Indivision ECS also works with the A500 but of course you won't be left with a colour composite output in that situation.
If the output of the A600 is for some reason monochrome, it's probably a PAL/NTSC issue. It should be possible to configure it as an NTSC machine (not 100% sure on the details here, never actually needed to do it).
EDIT: Oops sorry didn't see your later post. If the TV has no VGA input you'd indeed need an extra converter for that. Or an A1200/A4000, since the later Indivisions for the AGA machines have HDMI/DVI output.
