Speaker switch box (2 x 2 or 4) with automatic ohm balance / adjustment, has anyone got a design for it please?

Hi, I have a cheap speaker switch box connected to my stereo amplifier as it only had one set of speaker outputs. When the smaller higher power demanding speakers are switched the box causes the more efficient floor standing units to shadow the sound which is annoying. The reverse does not happen, but when all 4 speakers are switched on the sound quality drops noticeably, as it would since the ohm matching is not right. I could buy an expensive Niles box but I don't exactly know the ohms for the smaller speakers since they have had the main drivers replaced (the original foam cone supports hardened and cracked in the dry heat of a few Australian summers). I suspect one set on, and one set off is a simple enough device even I as a novice could design it, yes i can, but would just building a better switch box using higher quality switches solve the shadow effect? and I have no idea about the 2 sets on so any ideas - a internet search didn't help much but maybe you call these things differently in the USA?