Disclaimer: This is a part of the loaner provided by Inks, a good friend o' mine.
Not only Maxell is not a well-known IEM manufacturer, actually, I have never even heard of this Japanese company until very recently.
Regardless, one of the main features of HP-CN40 is its oval-shaped nozzle. The manufacturer claim that the shape of the nozzle ensures a tight fit within the ear canal. Due to its unpredictable nature in acoustic design, changing the shape of the bore has never been realized by any IEM manufacturers as far as I know.
PRO: Linear frequency response & low distortion
CON: Slightly short high frequency extension.
ON SECOND THOUGHT #1: CN40 performs the best when it is slightly inserted into the ear canal, possibly 3 mm away from the reference plane.
ON SECOND THOUGHT #2: Additional acoustic resistance further cuts the high frequency extension.
ON SECOND THOUGHT #3: Blocking front vents boosts the bass and the resonances located within 3~4 kHz range.
ON SECOND THOUGHT #4: Since frequency response shaping is already optimized at the oval bore, CN40 is not really hindered by a bore resonance. However, once the stock bore size/length is somehow radically altered, as seen above, the resulting frequency response shall become quite skewed, cutting off the treble extension.
ON SECOND THOUGHT #5: If the resonance at 3~4 kHz can be damped by 6 dB, CN-40 should become very close to the Hi-Fi reference target presented by Harman.