If I had 300 bucks I rather spend it on good power amp and make simple adapter with parts from radioshack.

You end up close to 0 output impedance,  very high damping factor, and endless power reserves.

You can make a simple adapter with no resistors or anything. Just need wire and a project box, female trs jack. Or can make just a ghetto female 1/4" trs to bare wire and function fine. If doing that depending how jack is insulated on outside I'd just wrap electrical tape around it for safety reasons since jack be sitting on floor.

If not, I'll just invest in good preamp. I use to use my kenwood c2 preamp as a headamp and had more then enough drive for even ortho's. I never used headphone amps really since all the good ones suggested is bit pricey and I can get a really good preamp or power amp for that money and do best of both worlds for speakers and headphones. Lot of poweramps are usually lot quicker and revealing and, have a flatter FR bandwidth per dollar if you know what you're looking for but, that's just me.

Also of course you're purpose. If ever plan on using speakers too then I just go power amp/preamp combo but just headphones I find it bit of a waste and just use a preamp. Power amps suck up lot of power too while even idling and just using such an easy load such as headphones your power amp is gonna just be chilling and be like "whatever bro, I go have a cold one while im trying to figure if this is a load or what that needs to be moved".

Up to you. That's just me.

In going your direction just audition some integrated amplifiers. Sometimes good integrated stereo amplifiers are dumped for incredibly low prices because people buy home cinema sets and do not know what to do with stereo sound.

The used market for used integrated stereo amplifiers for low cost prices sometimes has hidden gems you can pick up and take away. When you have good DIY-skills you can often service them yourself too, especially older series.

Well, these are rated 100dB/mW. Dunno about the ohmage tho.

Yamaha R9 receiver good start since they go dirt cheap usually.  Just have to use it correctly cause of the annoying "variable loudness" knob. It can sound warm if you don't go past 12 o clock so I figured out way to bypass that dumb feature by keeping the volume knob turn all way up and use the loudness knob as volume knob. Works wonders and sounds lot cleaner and natural.  Other then that very good one of few "receivers" I really like. Pulled it out earlier and fixed the wiring due to bad wiring in volume knob and was cutting out left channel all the time.

Really good receiver if you know how to use that silly variable loudness knob. It has class A feature but find no point using it. It just makes the thing run extremely hot unless you're running speakers in 3ohm nominal range since it'll run much cooler in class A due to extra current being utilized and not just dumping it as "heat".

If live in US nad 214 something consider since they come by cheap and actually well built discrete  straight forward power amp from them. Could match it eith good preamp or hook it straight to a source/interface and control volume that way.

Sansui makes good stuff and like them but cause of the name you're gonna pay a premium like Pioneer and marantz.

I think sansui though makes better stuff than marantz and pioneer.  Find marantz and pioneer gear over rated performance wise.

Harman kardon is good too made excellent stuff in the 70's/80's but can be hit miss in pricing cause they can be grabbed up dirt cheap, good price or sometimes go for super high especially the citation series and PM integrated series. The 700/800 series are usually under the radar but most people who has them keep them cause how well they perform and built.

Qsc is pretty good ajd they're power amps can be had for reasonable prices.

Just look around and give things a listen if you can. Best thing to do instead of blindly purchasing something.  Look around on craigslist or something if you can. I never used emotiva stuff so i can't say.