A bigger cab will yield more LF gain, so more for the room to add to, especially if tuned lower. This should cover most, if not all, of the LF boost from one side of the loudness control's 'smiley face' EQ and depending on which XO you have, adding a HF bypass cap will allow some portion of the HF through unattenuated to mimic the other side of the 'smiley face'. Otherwise a more adjustable XO will be required to get the proper amount of CD horn EQ.

Another option if it's ever in the budget is to switch to a matching impedance tube amp that contour shapes the speaker's response to a 'smiley face' one that the loudness control is designed to mimic except without the efficiency loss that potentially will audibly increase harmonic distortion if played loud.

GM