Why a headphone amp

I am looking to sell my setup and maybe get some nice headphones. I have theta enterprise monoblocks so it occurs to me why does one need a special "headphone amp" when I already have good amps. Can one connect headphones to conventional amps? Thanks
Most modern headphones will be driven to ear-shattering volume levels by inputs that are not much more than a milliwatt (0.001 watts). Your 300 watt monoblocks, although less powerful than that when working into the relatively high impedance of most headphones (compared to speaker impedances) would still be WAY too powerful, and would also provide voltage gain that would force you to use the preamp's volume control at just about the bottom of its range.

There are a few preamps that are suitable for driving many headphones directly from their main outputs. Although most preamps are not suitable either (unless they explicitly incorporate a headphone amp function), because of output impedances that are too high and/or other reasons.

-- Al
There are adapters that can be used,but why would you? The cost of a decent HP amp is not much more.
There are a handful of manufacturers that offer headphone 'interface' units that allow you to drive headphones from a conventional amplifier's loudspeaker taps. They are inexpensive (compared to a decent tube or ss headphone amp) and shouldn't be considered a permanent solution. But could be an an inexpensive way for you to try out headphone listening without spending a lot of money on gear.

Driving a pair of low impedance headphones directly from a conventional preamp can also work while you're just in the "proof of concept" stage ;~))
There are some people that use a poweramp for headphones. The amps are usually low power tube designs, not high power SS.
Bigsam2, I have already Benchmark DAC1 with good headphone amp built in, but I bought separate headphone amp for convenience. DAC1 is across the room and requires long cable that is easy to trip over. In addition switching off main amp would require fiddling with switch on the back of DAC1 and possibly switching to wrong full volume position (I use its volume control - connected directly to power amp) or switching off power amp, that does not have power switch. My wireless server is next to my listening position and the best solution for me was to buy headphone amp/DAC. I got OPPO HA-2 and it is amazing with Yamaha HPH-200 headphones (another gem). It has asynchronous USB, SPDIF, DSD and analog inputs (I use USB). It has headphone and analog out. It uses SABRE 32 bit D/A converter and discrete output stage. Clarity is simply amazing (better sounding than Benchmark DAC1). It is also portable (has lithium battery built in) and works well with supplied ON-THE-GO USB cable with my android phone. My main computer is Mac Mini. It does not require driver. $300 is a little high, but it had great reviews and looked beautiful - wonderful packaging (I don't work for or sell them, in case if you wonder).

Look around for a used Grado headphone amp. Get the battery operated one. A friend has one and loves it.
I applaud your inclination to "get some nice headphones." Personally, mine see far more action than my main audio system, as my wife is not an audiophile (imagine that). However, I have a long history of looking (and finding) ideal headphones. Your amplifier question has probably been well-answered by earlier postings.

My advice is simple: Try to attend an audio show. It will save you much frustration. They usually allocate a large room exclusively to headphones and headphone amps. Find the phones you like and invest in a quality headphone amp. Any of the phones you audition will be fine, but try to find one that you can live with long-term. I recommend Stax, but they are not for everyone. Good luck in your search.
Get a good tube unit they are cheap enough be a sport.Headphones will never take the place of a first rate setup.
For flexibility overall with headphones or speakers Bel Canto c5i is excellent all around digital integrated amp including for use with headphones. I have Sennheiser Momentum phones that C5i does a wonderful job with and it is all the hifi overall that most would ever need.