OK, so l have spent almost 2 months with these earphones and l think I'm ready to share my experience if anyone is on the fence as they are a pretty inexpensive, nowadays especially if you're in the U.S buying from Canadian dealers. With the conversion rate being as it is, it's a great deal.
Set up: Straight of iPhone 6, iPad Air2 and Dell laptop. No AMP or DAC required.
Simply awesome, small, solid, no cracks and clicks or sharp edges, cable is very nice, connectors are snug don't rotate randomly. Cable with mic is a bit tricky for first few days and won't stay still around the years, but after a good few days will settle down eventually. Mic is and inline control are great, clear and responsive - no issues there whatsoever. Shirt clip is nice too, tips: comply are included and silicon are not good, too thin and for my medium size ear-canals even the L size wasn't big enough, strange.
Those have a unique shape and come in sharp black or beautiful white they if you like white earphones, they are quite small but you need to fiddle around a bit to get the right fit. Not a big deal but something to keep in mind since they are small but comfortable IF you find the right feet. The sound is pretty flat and neutral with a bit of a bass hump for the RoomFeel. First 2 weeks it was uninspiring, nothing special really. I was using the stock tips and l thought l was getting a good seal, but in fact, l wasn't. With regular tips l usually use M size tips, with the PSB even the L size won't provide a good seal, they are flimsy and thin and just don't get the job done. With Comply the seal is very good but the IEM can stick out quite a bit and they sound congested. l had a set of old Meelectronics at home and they improved things dramatically.
l often like my IEMs a bit warm, the M4U 4 don't really provide that. They are rather flat and dry, not in a bad way, just the nature of the beast. Bass is solid, not boomy and rather punchy but in a loud speaker type of fashion (RoomFeel?) They are fairly detailed and soundstage is really good.
Isolation is really good and lucky l didn't notice any microphonics which like all of us l despise.
They work really well with most genres, ADM, jazz, rock etc... You name it, they sound true and accurate, if you are used to warm sound and V shape signature they may sound on the boring side to you. Details are nice and crisp but non-fatigue, highs are never harsh or shrill, if you like strong and rumbly type bass you may want to look elsewhere. While bass has some nice texture it has a rather unique type of bass that l have never heard before.
Things to consider:
Like any IEM's these may not be work for some. They need to be inserted rather deep and are VERY tip-dependen, I can't stress this enough. The shell sits in a way that if your ears are small it may not work well for you and may become painful. If you over insert them they sound muffled, if the seal isn't good they sound thin and weak.
Sound signature is actually really close to the NAD Viso HP50 full size headphones. You get a little more bass though and a bit more sparkle in the high frequency if used with good silicon tips.
I must admit, at first l was underwhelmed with the sound and overall fit. It took me a while to figure out the fit and find the right tips but once l did things obviously had improved dramatically.
They have a very interesting. Also, Paul Barton and Co. hand tune all their M4U 4 against the reference. This is really remarkable and in fact the first set of earphones that l hear and can really feel that they are well balanced.
While l bought and sold many IEMs the M4U 4 are keepers.
Highly recommended if you like balanced uncolored sound with a very unique bass.
Some great info from Paul Barton, Chief engineer of PSB Speakers about the M4U 4 and M4U 2 and the RoomFeel technology:
Edited by Tommy C - 3/20/17 at 10:08pm