It is not a matter of finding a receiver with "Pure Direct", you just need a receiver that takes an 8ch Linear PCM stream via HDMI.
This is not really so. Many receivers that accept an 8Ch linear PCM stream still perform processing on the stream that is not user defeatable and not desirable for an active crossover system.
The words "pure direct" are not important, I agree, that's just one of the ways that receivers describe a "passthrough" mode. Many receivers insist on applying some processing to the channels they receive and will not pass them through without additional processing. It's hard to tell which receivers will allow you to do that without physically having one in front of you, unless it's manual makes clear that there is a passthrough, direct, pure direct, etc. type mode.
I can guarantee that being able to accept an 8Ch PCM input is not a guarantee that a receiver will be suitable for active crossovers.
When I read the manual for your receiver, I don't see any reason why it should not work, honestly I think it is something in your setup (on the receiver or JRiver) that is not correct.
I agree that it might be possible for OP to configure his receiver to reproduce the signals, but it may not be too. I've encountered several receivers that insisted (for example) on applying a low pass and some spatialization processing to anything coming in on a "surround" channel. The processing was not in any way defeatable.
That said, I think it's worth OP's time to try and find a way to make it work with his existing receiver, but as they've noted several times, it may not actually work.