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!
RCA Splitter to convert a single RCA output into two
I would like to connect a single RCA output of the tube pre-amplifier to drive two monoblock amplifiers (instead of just one). Would it be "safe" to use the RCA splitters such as the ones offered by Audioquest without a risk of introducing any sound quality degradations or making a harm to the system? Is there any limitations on how many RCA splitters could be used to split a single RCA output? Would it be safe to chain the multiple splitters to split a single RCA output into three, four or even six RCA outputs to drive multiple power amplifiers?