Passive volume with direct switched inputs should be OK provided that suitable precautions are taken to keep crosstalk to a minimum and the selector switch is from a good brand . Interconnects should also be relatively short. Large capacitance and cross talk will kill the SQ.
Relay switched is probably better.
But never forget to try out an active preamp ( or buffer) . It could sometimes give you surprising results. It's not about which is 'perfect' , it's all about which end result your ears like !
I remember hearing a Mark Levinson preamp in circuit compared with the preamp out of circuit. While the preamp had a very flat frequency response , for some reason there was more bass extension with the preamp. It was never determined what caused that ! The load on the source changes with or without the preamp and depending on the 'impedance' interaction , it might sometimes help to have a preamp or an active buffer. Try them out.
Tube preamps. Not all sound the same and saying 'tube sound' may not describe what you might hear. For some people who think that tubes sound rounded off in the HF and so sound soft might be surprised to hear well designed tube preamps sounding extremely clear and bright and not soft or rounded off in any way. You never can say what something sounds like until you have tried it in your own system ! Might sound great at your place and terrible elsewhere ! It's a lottery !