i wonder if this approach will give you what you want. I have a pair of STA-9s though I tried using just one in stereo mode to compare the sound with the bridged mode. I was pleasantly surprised at how well the single unit drove my Magneplanar speakers. However, I found that the sound of the STA-9 in bridged mode changed significantly. Yes, the greater power output helped to increase bass depth and control and improve dynamics. Beyond that the sound quality changed somewhat in bridged mode: it became more valve like with better imaging, more detail and faster transients. I cannot suggest why this may be the case but I wonder if those same benefits will be forthcoming in your "vertical" configuration.
Also - specific moment - I have to use original connection link of speakers between upper an down contact groups. When I will
get opportunity to control 4 channels - I will remove this link and use 2 pairs of identical Acoustic Revive SPC Reference. And - additionally - I will remove as well intermediate connections between speakers contacts and cables contacts (those intermediate connectors hold the connection link entirely). It may be hard to imagine this construction without photos, but removing of such
contacting group from the path of signal will give me more preference.
Future is bright, great and positive.