Upon first look at the room picture, I exclaimed "Oh my God!"
the Spires have phenomenal bass
in that room!
Cover the TV. Add something to the right wall to reduced reflections. While the minimalist shelving has nice esthetics, it's a disaster for reflections. I didn't do the math, but the wall/ ceiling corners could be a problem.
Sensitivity: 91 dB/2.3 Volts/Meter.
Seems like the amp should drive to S T U N [110+db @ 3m]
unless it is giving up into:Nominal Impedance: 4 Ohms (0.8 Ohms at 20 kHz)
The bottom end impedance is 50-100x 20k. I've never been a fan of amplifiers with so many output devices per channel. Transistors are non-linear devices and into a complex load with 20 devices per channel, unless the devices and driver circuitry are extremely well selected, things can get ugly in a hurry.
Perhaps with the subs in the previous install, rolling the low end made life easier for the amp.
What speaker cables? Perhaps the amp does not like the ESL/cable load?