High quality Speaker switch box for multiple amps?

Looks like I would need to separate "boxes" for this. One splitter for the preamp audio outs and one for speakers

You would have to build a custom box. I recommend Grayhill switches if you want something that won't completely destroy the sound. You'll pay a bit for one but they are some of the best for something like that.

Try this one:
https://www.amazon.com/Niles-Black-FG00230-Speaker-Switching/dp/B00W4FN47K
It switchs input and output of 2 amplifiers, 600W rated, but only for single ended.

Van alstine has a box that can switch all inputs and outputs and level match them. About $1500.00. 

This is a link to Van Alstine comparator as bugredmachine recommended:
http://avahifi.com/products/accessories/abx-comparator-switchbox
And it safe to switching bridged Amp.

To make those Amplifier switcher safer for all Amplifiers, parallel all speaker outputs with 25 -100 ohm  5watts resistor.

if testing, you could also try running one amp into each speaker and using the pre-amp to "switch"

this won't test the sound staging and some other things, but costs nothing

Regarding a line-level switchbox to go between your preamp and the two or three power amps, and assuming that all of the connections involved are single-ended (i.e., RCAs), I’ve had good results with the DB Systems model DBP-2JAU/5 ($110) listed near the top of this page. I've used it in a couple of different 2-in / 1-out applications, but it can also be used 1-in / 3-out among other ways. It is similar to the DBP-2J/5 for which a review and description are linked to, except that it has gold-plated jacks. For contact information go to their home page.

Another possibility, although it costs $359 plus options, is the Decware Zen Switchbox.

Good luck.  Regards,
-- Al

3 amps is tough.  I see this one for two amps into one or two speakers:

https://www.amazon.com/TC-7220-Amplifier-Selector-Comparator-Crossover/dp/B00LNOAGZQ

I would use this between the preamp and amps (unfortunately, no longer made but can find used):

https://www.amazon.com/Niles-Black-FG00002-Auxiliary-Selector/dp/B00006I5J0

FWIW, I've had a standard Decware Zen Switchbox for a couple of years now and find it to be completely transparent when using it to add sources to my 1-input-only "integraged" amp(s). 

I finally found a perfect solution for this problem. I tried the Mapletree Audio switch box, which admittedly is a little more elegant, but I did some A/B testing and felt like there was a loss of that last 5% of resolution and substance. I do have a very resolving system. I don't think the difference would be noticeable with a mid-level system.

I looked inside and I have to assume that the puny contacts and hook up wire do not do the job (even though they use a high quality rotary switch). So I thought about a hard connection to avoid small contacts and skinny wire. A simple, old school knife switch works best. I felt like I got that last 5% back. I tested two big copper 4PDT switches and this one has the largest contact area and the most copper where it counts: Tatoko Knife Switch Dual power Copper Power Disconnector Switch Semicircular switch 4P 100A https://www.amazon.com/dp/B07V5Z6GP7/ref=cm_sw_r_sms_apa_i_J-8yEbBBR32DH

Remember, if you have a tube amp you should have resistors in the circuit so that the tube amp is never on without a load. Not sure what the load should be. Do your research. I used the resistors that Maple Tree sent with their box. Btw, anyone want a good deal on that box? :)