The Asian made, tube/hybrid headphone amp celebration occurred soon after the intro of the Musical Fidelity X-Cans. The Mk I and Mk II X-Can amps were in that cylindrical case; they sounded as good or better than they looked. The first similar circuit, Hong Kong sourced, hybrid headphone amp I recall was the Indeed brand, with the familiar looking, tube popping thru the clear plastic top, cute chassis. I bought one as soon as I saw it available, even before they offered RCA output jacks. I knew the headphone amp could be utilized as a preamp...I simply had to wire up a stereo phone plug to RCA plugs adapter interconnect...
Sound-wise, the first Indeed tube/FET hybrid headphone amp, Indeed Gen 2 and Indeed Gen 3 sound excellent. Numerous family members have followed my lead and enjoy their units. These are not simply tube buffers, they have serious gain and drive. They are true hybrids, with the tube and FET active, not only sitting pretty. Of course, that funky looking, red LED underneath and blue LED glowing around the tube can be hypnotic...
If an amp can drive 32 Ohm headphones, like Sony, think how easily that device can drive higher impedances with lower distortion ! The Indeed could even drive my ancient 4-8 Ohm Koss and Nakamichi 'phones. Driving a line level input will even be easier and will definitely function to "sweeten the sound" when connected between a CD player or other digital source through your solid state or tube amp. With your digital sources, these hybrid headphone amps can certainly function (very well) as a tube buffer.
I have used the Gen 2 as a stand alone preamp, driving tube power amps, and can vouch for its' abilities and sound qualities. Note that the Gen. 2 and Gen. 3 have switchable inputs, RCA jacks or stereo mini-phone jack, PLUS switchable filament voltage, 6 or 12 VDC, for 6DJ8 or 12AU7 tubes ! Did anyone say "tube rolling" might be fun ? Well, it certainly is nice to listen with and fun to decide on the tubes you prefer...
As Indeed welcomed in the market, within a year, suddenly many brands appeared. Some sound great, but some do not ! Then, that Bravo company decided to copy the Indeed product, as in the Little Bear. They can sound nice too, but beware of the switching power supplies. Some of these "can" amps use switching mode power supplies, which emit too much EMI/RFI to nearby, high gain circuits like phono and mic inputs. The Lil Bear caused hum with a nearby record player, while the Indeed did not cause any problems; except having to spend more money for the Indeed products.
I can heartily recommend that this $50. to $100. expenditure, delivered to your door, of a genuine hybrid headphone amp, can be well worth the trial. At the very least, you will have acquired a sweet sounding, tube/FET headphone amp which can drive low to high impedance headphones (now known as "cans"). However, their worth is probably even greater as an "audio sweetening" buffer with gain...ENJOY...