70$ vintage amp doesn't sound ridiculous.
I would suggest even cheaper solution if you need more bass, impact, 'dynamics' or warmth... EQ.
In other words - if you don't hear clipping, lack of transparency, speed or significant soundstage congestion, there is no need for ultra-high-watt poweramps or super-warm vintage gear. I really don't know what all this talk is about - there are people who prefer low-power low-current Vali over ultra high-power high-current Mini-X just because Mini-X is too bassy while Vali is more transparent with only slight warmth. There are some who find Lyr to be too warm and bassy... And there are some that praise Lyr for delivering unbeliavable dynamics (in other words, moar bass) which cannot be matched with something like O2 (of course, because O2 will not deliver bass into anything including IEMs in a more than neutral manner). To continue - Vali or Mini-X can have smallish soundstage which some folks dislike... Alright, that's something not easy to fix with EQ they move higher in price because the better gear, the better soundstage rendering (that's the only thing that I have really found to be true universally)... But still, it's not about power or something like that, it's about quality (which can be delivered in 10W per channel as well as 0.5W per channel). My DA8 out of balanced XLR outputs provides the most expansive soundstage I have ever encountered (every headphone you throw at them immediately become expansive-sounding, not kidding), very good imaging and transparency even though it does not provide tons of current or Watts at all. This xlr output is less powerful than the DA8's headphone out and yet it sounds significantly superior in soundstage as well as rendering of microdetails. Bass is more present with headphone out but appreciably more detailed and tighter on the less powerful XLR out.
Yes, if you tend to listen loud, you need more headroom and more current to meet the demand with difficult-to-drive headphones AND WITH difficult-to-reproduce music. But for normal listening levels and normal music, almost everything will do the job for the vast majority of headphones (including HE-4) as long as the amp does not sound like crap itself and is not underpowered seriously (it should have at least like 0.5 to 1 W to become versatile enough). It's much more about what one's ear find pleasing than about power or 'power reserves' or whatever. The way I see it simply - more powerful amps usually provide more bass with orthos... So if you are after bass, you may find it useful. But I am not a basshead... So I don't care much about it, HE-4 have plenty of bass with everything I source them with music from. I can make them more bassy using EQ but it's not neutral for me anymore (HD800 and HE-4 lover here).
I went through the whole HE-4 thread as well as some others and this is what I've gathered... That it does not matter whether it's ultrapowerful or normally powerful amp or how great their power reserves are - but rather about it's sound signature, about the tubes used, about what each listeners is seeking for.
Check HE-4 demands... Do you see jiggawatts of power needed to listen to music at normal listening levels? I don't.
