Try and find a pair of Ion Systems JC 1A speakers! They are the same size as Kans or P3s, ported to the rear but the speaker is nearly fully stuffed with wool so the port doesn't do too much. The crossovers are external and live in a little wooden box that sits above or below the speaker. There are some pictures in the main Nytech thread.
If you manage to find another pair of MA40s and some active boards you can drive the speakers actively.
The JC1A speakers were highly regarded in their time and Naim also recommended them.
If you can't find a pair of JC1As then perhaps a well cared for pair of ARC050 speakers (designed to work against a wall) might fit the bill.
As you no doubt already know the MA40 can drive nearly anything with ease, so perhaps look for some classic speakers that were considered hard to drive, Acoustic Energy AE1, Celestion SL6 ?