Input impedance should be at least 10 times higher than the output impedance. It's half of what the original specs in the manual say with a y-splitter and gets less the more you connect in parallel. Y-splitter normally is safe.
You would need an active splitter if you want to connect more than 2 in parallel. If the impedance ratio is too much off you would hear first some changes in the frequency domain (too bright etc) and in the worst case the pre couldn't drive the amp anymore to its full output power . No need to spend a ton of money for a y-splitter.