Unfortunately the Massdrop website didn't give me the option to write a review. Perhaps that is because I didn't purchase through MD and therefore am not a "verified purchaser?"
At any rate, I really like this unit for what it is: A very tiny speaker amplifier with moderate amounts of power and good sound for the price. Do NOT think of it as a headphone amp, consider it a speaker amp that happens to have a headphone jack as a convenience. With these expectations, I think you'll enjoy this product.
I am using it in a bedroom, to drive a pair of cheap polk tower speakers and provide amplified sound for the bedroom TV and blu-ray player, without taking up space or being visibly prominent as a receiver would be.
It drives the 6.5" Polk towers to very loud levels, has surprisingly impactful bass, and doesn't sound strained until it is at very high volumes.
Best of all, it fits UNDER the screen of the TV. It is much smaller than I anticipated, which is a very good thing for my application.
The bluetooth receiver works great and allows me to stream music from my phone to the speakers, and I have had no issues whatsoever with either optical input.
If you are seeking a headphone amp, look elsewhere. If you want a tiny speaker amp for non-critical listening that packs a surprising amount of punch and has a wealth of connectivity options, I highly recommend this unit.
*** UPDATE ***
I like this so much I bought a second one to use at my computer. I have it driving a pair of Klipsch reference series bookshelf speakers, and feeding Klipsch 10" sub via the SMSL's subwoofer line out.
I just cannot believe a Class-D amplifier sounds this good. There is space around the instruments, an open, airy sound, smooth and crisp highs, punchy dynamics, and a remarkably thumpy and tight bottom for the size. My Klipsch R-15M speakers can sound slightly bright in the upper mids where I had to pull a couple dB's out via EQ on my prior amp, but it is super smooth via the SMSL without losing any presence. And there is a genuine sense of 3D imaging with near-field monitoring.
I hated the sound of the SMSL M8A/9038, but I love this product.