I am going to use 2 preamps but wondering if using a switch box for all the inputs and then switching them to either preamp, or using splitters from each input and running 2 cables , one to each preamp .What do you think is best
Are you thinking of using the splitters to connect each of your sources to both the preamps at the same time ? If you are then there a couple of things which might cause trouble:
Ground loops: you'll be creating quite a few of these. Without knowing the exact details (including the internal wiring of the sources and pres) it's hard to predict whether these will be a problem. But they might.
Source loading: most modern sources can supply a decent amount of drive current and most modern pre-amps don't draw an excessive amount of current from whatever's driving them. But there are exceptions to both of these rules and if you happen to have one you may find the signal becomes weak and/or distorted. A particular issue can occur if one or other of the pre-amps shorts its unselected inputs to ground. If you're trying to use a shorted signal line to feed the other pre then you'll find it has zero volts (= no music) on it.
A switch box would, in principle, avoid these problems. But you'd have to switch the grounds as well as the signal lines if you wanted to avoid ground loops. And what will you do about the pre-amps' outputs ? They might need switching too.
If for a multitude of sources your first option would require less stereo pairs of cables. I've used both methods in the past in various roles & both seem fine in terms of SQ TBH.
My MF A1 fbp pre-amp has a couple of unity gain inputs that can be configured for either line-level or HT/Unity gain so I can use my Audiolab 8000AV processor in a variety of roles. ie a multiple input switchbox for extra inputs, a straight 2ch pre-amp or a full on AV/HT processor or an additional DAC with HD ceedee decoding even.
I have a pro switch box that has s video connections aswell but the audio has 4 inputs and 2 outputs .You can select any of the 4 inputs to either or both outputs so i am going to give this a go and see if effects the sound quality first , :^ thanks for the heads up with ground loops