A speaker selector is a simple passive device. The only reasons it would need anything special is if you want to have multiple speakers on at once and your amp can't handle the lower impedance or if you're pushing huge wattage.

Output selectors such as the Mackie Big Knob and the SM Audio M-Patch 2 are designed to work at signal levels and not loudspeaker levels. Anything that uses a passive volume control is not going to work.

You can buy loudspeaker selector push-button boxes that are simple switches, and these should work (at least for a while). However, many amplifiers may object to the momentary open circuit between loudspeaker selections, and I personally would not risk using a Quad 405 on them when powered on. They would be fine if you power off a vintage amp before changing the speaker selection.

The switches are essentially zero ohms, so they pass on the loudspeaker impedance to the amplifier..

Thanks. I have one of these:
https://www.allekabels.nl/audio-schakelaar/514/1039940/audio-schakelaar-stereo-tulp-6-weg.html
...but those are the wrong connector plugs but still I wouldn't dare fitting this between the Quad and the 2x 2 monitors. I used this a while for routing a CD-player, tape deck and some other simple stuff, and that went fine but those are all low levels indeed.

This one at least has the correct connectors and it's 8 ohm, 100 Watt max, so it seems to be made for higher levels:
https://www.velleman.eu/products/view?id=378030
https://www.allekabels.nl/audio-schakelaar/514/1075272/luidspreker-schakelaar-klemaansluiting-4-weg.html

I dont want to sidetrack the conversation, but are the active aurotones out of the question?

Ive read a reveiw in Tape Op on the new version of aurotones, and the reviewer uses this cost amp for his and is happy. Sorry to sidetrack, just thought it may be a useful alternative, and maybe even cost less.

Fyi the reveiwer uses the "+" model of this which is now discontinued, but was at the same price point as this one is currently.

https://www.amazon.com/dp/B01FZKA28Y/?tag=r06fa-20

look for a used SPECO/CSI SLS-6 on E Bay or Reverb.

"Selects up to 6 pairs of speakers No-power-loss transformer impedance matching system Quick connect terminals will handle 6 to 22 gauge wire Left and right channels are completely isolated for safe operation with any amplifier Handles up to 180 watts RMS power (30 watts/location)"

i have one of these. i used to use it to matrix switch between my aux speakers (NS10 and Auratones), powered a Haffler 150 watt transnova amp. worked for years no problems. since it uses transformers instead of resistors it won't have the issues Boswell mentioned.

if you can't find one i'll sell mine but shipping will cost as it's pretty heavy.

Hi-fi folk have used speaker switching boxes for years, but in essence it's a double pole, double throw switch in a box. The simplest of circuits to wire up, and as they break before they make, very amplifier friendly. Most of the expense is the connectors and a posh box - not the actual switch. If you fancy a project - it's a good one to learn how to solder. Nice chunky wires, big terminals - little really delicate work. Could be fun - or you just spend six quid so that's what. 8 dollars? on one of these.
https://www.reichelt.com/gb/en/speaker-switching-box-2-ways-gc-5529-2-p203845.html?PROVID=2788&wt_guka=54951022235_263550069569&PROVID=2788&gclid=CjwKCAjwnMTqBRAzEiwAEF3ndn8sRz2w5zepBdsm7IbBDu4pTh7RhK3JDxbx10BbfblT4sDronR3FBoCHjkQAvD_BwE&&r=1