Y connector from pre-out to powered sub and...

This will not present a hazard, but it will be very slightly less than ideal sonically - just a very slight (and I believe quite tolerable) whitening up of the highs in the CJ. The determining factor will likely be whether or not the CJ actually has enough juice to to drive both devices from the same output. Very often consumer gear ends up lacking enough drive to make this sort of thing work, pro gear often has less trouble, but sometimes you run across HiFi gear with enough voltage. You'll very likely simply have to try it and see. The two loads will not really see each other, rather the CJ will be seeing both loads at the same time. If it all sounds perfectly acceptable, except for the minor quibble with the highs as noted above, then you should be good to go. If the CJ isn't up to the task, then it would only likely be a matter of the sound quality of either, or both, of the add-on equipment temporarily not being up to their usual standards for you...there should be no harm to anything being done at this point, but it will be telling you that the CJ is being asked to do more than it can do and that this idea, at least for these two particular add ons, won't work. Just remember that the reverse application of trying to feed two outputs into one input is not a workable idea...this can double the load going in and can create an overload condition that can sometimes cause things to fail (whether outright or over time) but splitting is not considered hazardous, it's just that it's sometimes just not feasible.

You will need to find out the input impedances of all the equipment to ensure you don't drop below the minimum cumulative output requirement from the CJ. Tube pres are usually not very flexible or forgiving in this regard.

Agree with Spinaker. Had similar issue with my ARC Ref 3 and 5 linestages. Pull my threads for more complete explanation and fix. I suggest, as did I, that you call the factory and ask for their advice. The issue is whether you may be overloading the pre's outputs. Btw, in my case, I would have shorted the outputs, so I needed a buffer.