I have a vague familiarity with planar speakers after living with them a few decades. I usually quote anechoic as it is the worst case and my room is actually heavily damped. The calculator does include 3/6 dB boost based upon near a wall or in a corner. It is a rough guide and has more options and more info than some of the other online calculators. Like any calculator, for anything, GIGO applies.
For those who aren't following our banter, the main difference with planars is they radiate in a figure-eight pattern to the front and back whilst conventional speakers radiate more or less equally in every direction from the front. Planar speakers (over roughly the midbass region depending upon panel size) do not radiate much to the sides or to the top and bottom -- it is a wave straight ahead and straight behind. This is a plus or minus depending upon how they are set up and how much you like room reflections contributing to the ambience. The back wave, if not damped, increases room ambience but also leads to comb filter effects that can mess up the image. I usually damp the back wave unless they are in a big room well away from the back wall (for any planar be it planar dynamic, ESL, or ribbon).
Microstrip's point (feel free to correct me, like you need an invite
) is that due to this radiation pattern, the sound from planar speakers does not fall off like a normal speaker, anechoic chamber or not. There is less loss with distance than with conventional speakers, more like ~3 dB instead of ~6 dB for every doubling in distance. a rough guide would be to use the "near a wall" approximation to model the effective SPL boost from the radiation pattern.
Nit-picking aside, what matters here is that the 400+ W from the amplifier should provide even more SPL than my rough (and roughly worst-case) estimate. I am not sure where the distortion is from but strongly suspect it is not the amplifier, more likely something upstream or in the speakers themselves. I remember troubleshooting distortion at a customer's house ages ago and it turned out to be coupling from speakers to TT/cartridge -- that was somewhat a pain to fix; we finally physically moved the TT to a null point in the room and used a different suspension system.
Swapping the wall power point might help tell you if the amp is being current-starved but I am inclined to doubt it. Remember a 3 dB reduction in SPL means 1/2 the power needed from the amplifier. Average listening levels for most folks only requires a few watts. The peaks can easily hit 100+ W, however; long-ago studies showed peak to average levels in music are around 17 dB, a power ratio of 50:1. Movies tend to be even higher.
FWIWFM - Don