Beginning of a new journey...

I had never been a headphone guy, up until very recently. My main rig in my home office is built around Harbeth P3esr, Rogue Audio Cronus Magnum II, Cambridge Audio Azur 851D, and Bluesound Node 2i. I'm extremely satisfied with the performance of this system, and cannot ever imagine parting ways with the P3esr's. They tick all the right boxes for me.

Having said that, my headphone journey started with my wife buying me a Bose Soundlink pair of headphones about a month ago. I put those on, and the first word that came to mind was 'meh'. I tried them with and without amplification, but the result was always the same. Meh! I concluded that maybe headphones were not for me. But then a friend of mine told me that Mass Drop (now just Drop) were running a special on the Sennheiser HD6XX headphones. With the first-timer discount, and free shipping, the cans were going to run me $175 + tax. I figured why not. I am in love with the Harbeth midrange and smooth sound, so 6XX's sounded like the right choice.

Now, I also have a Naim Unituque (V1) that was in my system before I upgraded to the Cronus Magnum. The display light had started to fade and eventually went out completely. So much for Naim quality! Anyways, I was hoping that the Naim would be a good match for the HD6XX's. But nope! Not at all. The sound was anemic, slightly better than running them straight from my iPhone, but not that much better. The bass was almost non-existent. I guess headphone amplification was an afterthought on the Unitiqute. I wasn't sure if it were the 6XX's, or just that the synergy with Naim was not optimal.

On a whim, I hooked the cans to the Cronus Magnum's headphone out, and immediately they came alive. The bass was much tighter, the midrange, vocals, treble -- everything was noticeably better. I can see why these headphones have achieved such a legendary status in the world of affordable hifi equipment. They definitely scale with better equipment, unlike the Bose. So now, I need some advice regarding a good headphone amp. Given the limited room on my desk, I would prefer something with a smaller form factor, and a single box solution with preamp/amp/dac. It will be a bonus if the unit can have a built in streamer, but not a must. I want to stay between $500-800, new or used. I'm sensitive to high frequencies, so tubes will be my preference but don't want to rule out smooth sounding SS amps.

Any advise is greatly appreciated!