I've got a little
Total Airhead from Headroom that I bought 10yrs ago to drive Etymotic ER4S in-ears.
I was looking for a good sounding but small headphone rig for traveling that was decent enough to use all the time. I ended up going with Etymotic ER4S in-ears that are great sounding phones but need more juice to drive than amps built-in to most equipment provide. I bought them from HeadRoom and they gave me a package price for the phones & amp within my cost constraint (can't remember what I paid, something like $350 for the package). It's a small 2-1/2 x 3-1/2 x 1" unit that runs on 3 AAA's, and is strong enough to drive the 4S's well for everything but monitoring while recording really loud music. It works perfectly for what use it for & I just wrap the Ety's and a 12" input cable around it and store it all in a zip lock sandwich bag. It includes HeadRoom's switchable HRTF circuit that can improve the soundstage of alot of pan potted stuff but never sounds good to me for live recordings. I actually tend to use that more as a tone control since it seems to pad the high treble and boost the bass when switched in. It sounds very good for it's price, but I do have the 'Total' version that came with upgraded components. The volume pot is scratchy sounding when adjusting it but has held up for 10 years. I only wish it had a low battery / clipping indicator. It sold for around $100 then.. I'm sure it wouldn't compete with a $1k headphone amp, but I've never heard a $1k headphone amp and I'm half scared I'd be spoiled for life afterwards if I do
.
They added a second headphone jack a few years later & I think they still make something similar in a slightly different housing. I think they also make one now with a built-in USB DAC.
If your a DIY type there are plenty of plans & a few kits for rolling your own in a candy tin.