90-degree coax connectors are not a good thing. Better is a sweep corner. A "Y" cable will work fine and they are very affordable, but you have lots of buying choices.
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?