I think anyone considering on spending in the sub $400 range should consider auditioning the PSB's.
I'm far from a bass hungry listener so I do find the M4u4 pleasing in "deep enough" bass reproduction. Overall the musical presentation is very good even if you do not consider the cost of the headphones w/ replaceable cables.
For an entertaining audition I'd urge anyone to test a B&O Beoplay h3. A cheaper earphone with thinner guage non removable cables. The "air" for a semi open headphone seemed noticeably larger in soundstage with a much more cleaner presentation. I actually hoped the PSB had that identical sound performance. The catch is I'm wanting robust earphone with sound isolation with removable cables. The H3 has a rich presentation with neutral bass which I'm after.
The M4u4 is a solid performer that appears to be built to last. Infact I deliberately used my iPhone 6 plus as a source using Tidal stream. This is my intent of these earphones for portable leisure listening.
I also compared the Shure SE425 and found them to be "dull" with a lack of dynamics. The clarity seemed to be lacking while I listened to tracks of jazz vocals from Melody Gardot. I was not impressed with the sound of the SE425.
In the same "testing" I tried a pair of RHA T20i. A darker heavier sound but for a "Bass Head" they'd appreciate the added eardrum damaging effects these earphones can provide. Clarity was much better than the SE425. The SQ seemed to be almost on par with the M4u4 earphones in the clarity/neutral department but a tad darker with much deeper bass. This is not my criteria I was looking for so the audition of the earphones ended 2/3 into listening to Diana Krall's "California dreamin'"
As anything goes for headphones/earphones it all comes down to personal taste. If it wasn't for the delicate design of the B&O h3 non removable cables I would purchase those compact beautifully fitting earphones for my application. The PSB's are on my shortlist that is cheap enough to grab for giggles but SQ is a downgrade from the B&O H3. For such a portable "transducer" I find the PSB m4u4 much more pleasing than most of the portable head gear like the Senn Momentum 2, Aedle VK-1, Audio Technica ath-m70x etc. Going from memory (frightening to say...) the Denon AH-MM400 is the only portable headphone gear I'm also considering for "portable" use. The laws of diminishing returns is present in anything we buy. The PSB M4u4 is a sweet spot IMO for sub 400 buck price point and sonics.
Great impressions...I would however expand your price range to the $500ish region...I prefer these to the W40 & SE535. They are that darn good to my ears!