As the title suggests: when and how is it a feasible idea to place monitors on their sides?
On a related note: since Yamaha HS8 monitors pump the lows out of their backsides, where would you place high-mid frequency absorber panels on the wall behind them?
It doesn’t say if the HS8 can be placed vertical, not all monitors are designed to be used vertically.
Just try and see how it sounds.
Keep in mind that you should place the monitors so that the tweeters are at ear height.
The manual also refers to positioning the monitors at least 1,5 meters away from any wall.
I recommend to just test multiple distances and see how the low end response is.
Just don’t use the same distance from the back wall for the sidewalls. Also avoid the same distance for the woofer for ceiling and floor.
As for absorption, first get a good as possible response without any room treatment.
Do some measurements and see what and how many bass trapping/acoustic treatment you need.
For more info an monitor placement https://www.genelec.com/documents/publications/Monitor%20Placement%20in%20Small%20Rooms%20-%20Resolution%20July%20August%2009%20Supplement.pdf