looking for an integrated amp in the $1500 range

Rogue Sphinx sounds like a great idea in the budget!

High efficiency equals good listening at low levels minimum of 92 for low-level listening.

The Buchardt are great-sounding speakers. IMO they work nice at medium to high listening levels. Surprisingly they can sound very good in large rooms.

OP,

Agree with what some have said, higher efficiency speakers might help. That said the Buchardt S400 MK1 already has a "mid bass hump" that works well, but takes some guts to get that passive radiator cooking. I'd suggest sending Mads Buchardt an email to ask his opinion info@buchardtaudio.com

I had the S400 mk II speakers, which are rated at 87 dB, in my bedroom. During low-level listening sessions at around 45 dB at night, I found that, with well-recorded music, I could discern articulated bass notes and open, airy treble without any difficulty. To maximize my listening enjoyment, I would activate the preset EQ in Foobar2000, adjusting the SPL to match a spectrum of multiband frequencies according to F-M-based equalized loudness curves (as described below). It’s important to note that if the resulting dB levels at certain frequencies approach the distortion threshold, it’s advisable to lower the entire spectrum of dB levels by the same amount.