I think you'll need an active splitter system if you want to run into 8 preamps simultaneously so you have a strong enough signal at the input of your preamps.
Just thought I would let you know that I know where you're coming from as I also enjoy using different preamps to shape my tone, especially for my home studio rig. Pedals are great, but having the tone sculpting available in a full preamp is hard to top.
I had several rack units which I've accumulated over the years. I wanted to be able to run my rack-mount compresor and eqs with different preamps, and switch between different preamps into my amp. Switching the compressor and eqs in and out of a preamp is similar to what you're thinking of doing as they may all be running at the same time, but switching between the preamps is different than you as I only want to run through one preamp at a time.
For the compressors and eqs, I couldn't find anything readily available, so I built a box with an input/ouput with 4 effects loops, with toggle switches to switch the loops in/out, as well as relays to use footswitches to control the toggle switching. Pretty cheap and easy to do if you understand basic electronics.
I planned to build a second box to switch the preamps, but then I found some NUX PML-2s for cheap which have an input/output and a switchable A/B effects loop that you can control with footswitches, so I used these units in a cascaded configuration. This setup allowed me to switch between my amp's preamp and 3 other preamps, and add the compressor and/or eqs.
I've reconfigured my rig a few times over the years. Right now, I've dropped the EQs, so I use my homemade box to first direct the bass signal into my amp's preamp or send it to the NUX units controlling the other 3 preamps. The homemade box also switches the compressor in/out, as well as my passive DI, and a signal mute. With the cascaded Nux units I can switch between my SVP, BBE and Ashly preamps, as well as the pure bass tone. I also use the footswitches to control extra features on the preamps, such as the SVP EQ or the BBE Sonic Maximizer.
I guess it's a little convuluted, but not really complicated - just some switches, relays and patch cords.
Here's a few pics. First pic is of the rack. Top to bottom is a tuner (always on), Rane compressor, Ashly preamp, BBE preamp and SVP preamp. On the bottom right is my homemade box, and 2 of the NUX units on the bottom left (there's a 3rd NUX hidden that switches to the pure bass signal)..
Second pic shows my pedalboard with the bank of BOSS footswitches used to control all this stuff (the switches are actually on a second lower pedalboard. Usually, I just grab the smaller top pedalboard along with a small amp when I go out to play.)
Third pic is of the whole rig, along with a few other goodies I use. Sounds killer, especially when I combine it with my old Mesa 400+ and the 2x12 cabs. It doesn't leave home very often, but I do drag it down to the barn every so often when we want to really jam hard and let all the neighbors know that the boys are back in town.
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