So, a similar experience. For me, it was interesting to realise that, in my case, most of the potential power of the amplifier is not used as I never go louder than -20dB, which would correspond to 2.8V FS for the AHB2 or 1W for 8 ohms speakers.I have a similar set up with TV (toslink) + Streamer (coax) going into the E50 (also used to control volume), then Audient id14 MKII as a gain+dsp filter device (knob at 2 out of 10 so roughly 6db gain) and final output to Presonus E8 sub + E8 XT.
with this setup I usually listen to -30db (I haven't measured it but SPL should be in the [70-75]db range).
I do hope that the dB readings of the E50 are according to the standard formula 20*log(A), where A is the attenuation. It would otherwise be a very strange design choice to show dB as unit but not to follow the formula.But I think that the E50 volume implementation is not linear (I've never reached the -15db volume level for example) which may explain why SPL at -40db is very low.