I think you have your answer: off-axis listening is probably responsible for this low volume effect you describe.

I wanted to chime in regarding getting the amp boards recapped- this is great advice. While I haven't yet done M33s, all of the DSPs I have recapped produced a massive improvement in the quality of the sound.

Congrats on your purchase!

Theater: 861v4_LPS, 818v3, HD621, Oppo 203, Lumagen Pro, DSP8000SE, DSP5500C, SW1600x2, DSP5500, Epson 6040UB, PS4
Living room: 861v4_LPS, HD621, DSP8000SE, DSP5200C, Oppo 203, Lumagen 2143, 818v3, Sooloos/MC600 Bluesound Node
Bedroom: M80
office: G68D, DSP6000
Garage: DSP5000
Storage: M33s, DSP33s, DSP5k, 501, 518, M1500, G98DH, SW1600