The Sony "works" because they planned it out with some thought.
Most duplicated components cannot be chained as successfully as the Sony's.

The RTI T1 or T2+ used with the RP-6 can provide a solution for the multiple changers, DSS boxes, etc.

IMO, the RP-6 blows away the RF base of any other manufacturer because of sheer flexibility and programming power.

I haven't used the MX-850 / MRF-250, but I did get bitten on the backside by the MX-800 system...

I have heard very good things about the new Universal products from this forum and other people I trust, and it is certainly cheaper than a similarly configured RTI system.

If you sticking with your Pronto NG, use Ernie's solution, it will work flawlessly.