The issue, in my opinion, has less to do with the Y-connector itself (as long as it's decent) than it does with the impedances of the amp and sub. With the Y-connector, their input impedances are paralleled and, if equal, the preamp sees half the load as with the amp alone. If the preamp can handle this (and most can), fine.
Another issue is the relative impedances of the two paralleled loads. If one is significantly less than the other, it will shunt signal current from the other. Again, this may or may not be a problem depending on the values and the preamp.
I, for one, use a preamp with multiple individually-driven outputs so such issues do not apply.
Another issue is the relative impedances of the two paralleled loads. If one is significantly less than the other, it will shunt signal current from the other. Again, this may or may not be a problem depending on the values and the preamp.
I, for one, use a preamp with multiple individually-driven outputs so such issues do not apply.