GTP, on at least one point there appears to be a misunderstanding on your part or in sources you may have read and are relying upon: there is no "brickwall type high pass filter". There's a low-pass filter of that type operating to cut off response above 100Hz in some models(e.g., my six year old EP500)and 150Hz in others. "Room gain" occurs in the lowest 20-40Hz octave or slightly higher in very small rooms; the low-pass filter in question has nothing to do with this. Some models do offer the ability to switch in a subsonic filter for frequencies below about 20Hz to prevent subsonic disturbances, such as rumble when playing LPs, from creating a problem.
