RCA Splitter to convert a single RCA output into two

The splitter should work but your tube preamp's output will be driving a lower and lower effective impedance as you add amplifiers. That could pose an issue, especially for a tube preamp because they tend to have high-ish output impedances.

An rca y - splitter will work fine - one per channel - from your tube preamp's output jacks. Save your money and buy a pair from Monoprice. No difference in sound whether you spend $3 or $300!

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.

Jason, now someone will kvetch about the price range you posted.  You ought to know better than to declare that around here. ;)

"Obviously a 3$ 'Y' splitter is Not going to be comparable to this 250$ version our company has just introduced into our line of Cable Concepts...."

(...just fulfilling my apparent duty as local PIA about audio faux paus....;)...)

...besides....you might be interrupting someone's efforts to put Child X into Yale, or that new BMW to impress the investors.... ;)