Cheap, Works Well, Fragile Paint Job
Purchased the HA400 to replace an old battery-powered Radio Shack earphone amp/splitter damaged by leaking batteries Also compared the HA400 to a Behringer MA400. The HA400 is around 2x as loud as the MA400, and also louder than the RS amp/splitter. I play wind synth with a church choir, and get adequate volume into good quality earbuds with synth and HA400 volume knobs at 60-75%. In a quieter setting, I have not noticed any hiss or other noise. Basically, the HA400 does what it was designed to do pretty well, but there is room for improvement.
1. If you use cables or headphones having a 3.5mm plug, be absolutely sure your 3.5mm-1/4" adapters are of good quality. I have a dozen of them from various sources, and half made poor contact, resulting in drop-outs or hum. This might lead you to believe the HA400 is at fault, when it's the adapter. A way to improve the HA400 would be to add 3.5mm jacks or substitute a couple for the 1/4" jacks. The MA400 has one output jack of each size.
2. A mono/stereo switch would simplify using the HA400 with mono sources. The MA400 has a switch.
3. Biggest criticism - the HA400 finish is an attractive satin black, but it seems to chip with the slightest bump to leave bright metal showing through. The steel case was apparently not primed, or it was improperly prepared for painting. I noticed this even before actually using the HA400 on the job. Extra care is needed unless you enjoy the appearance of "distressed" electronics. Powder coat would be a lot more durable.
In all, the HA400 is an effective, inexpensive solution for distributing a signal to 4 sets of headphones that should last. However, you might need to treat it with kid gloves to maintain the finish, regardless of whether it's used on the studio or on the road.