Random Q: are the focal clear over-the-ear or on-ear? If the former are they comfy? I have Akg-k702 which are huge which I love (the fit). Perhaps my ears are unusually big 
Try the HD800S with the EVO 150 and see how it works for you. The HD800S is a 300 ohm headphone, and doesn’t need a ton of power to run. An external headphone amp would likely refine performance of the headphones, since GENERALLY, the built in headphone amps are not that great.
Depending on your budget, there are some fantastic headphones amps available in both tubed and solid state variants. Have you actually heard the HD800S? If you’re a basshead, it wont be for you. HD800S are very comfortable, lightweight, and have aftermarket pad options. Another great set of headphones at a very attractive price are the OG Focal Clears at less than $900 at headphones.com
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I have five headphone amps in service. All of them are connected to “preamp preouts.” —Say that fast three times… Some are connected via balanced, some are unbalanced. I’d be interested in why you believe using a preamp to power a headphone amp is a poor choice.
Cheers…-----Robert.
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The Cambridge Evo 150 is 3250 USD at Crutchfield. It better be good.
I love my used Chord Mojo … 300$ on the used market.
It’s an all in one device. All in one devices have tradeoffs and shortcomings.
I took a look for the Headphone amp specs, and they weren’t readily published.
I’m sure it will be a good headphone amp, just not as refined as a good dedicated one.
That’s true in general. I prefer separates myself, although I’ve never had separate power amps, only integrated amps.
If you can hear a difference without listening with your wallet… You can spend thousands of dollars on refinements that don’t really make any difference.
As per my last post.
If you are happy with the solution then go with it. 
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To be fair, I’m not running anything from an “amp” using pre-outs. Strictly preamps.
Yes, you can, however, that adds length and has the “weight” of the adapter and the 6.5mm connector all hanging off the 3.5mm socket in the amp, no telling how long that will last with regular use. I have had bad experiences with this set-up and would not recommend it for very long time use. especially when it is likely to be the $3500? amp that would be the failure point. YMMV as usual… 
Yep, one of those tradeoffs or shortcomings would be the size of the headphone connecter and could be the quality of the signal to headphones. Some manufacturers throw the headphone jack in as an after thought hence needing to use a small not so robust single ended connector size. But as I said earlier I have no experience with this particular device so not saying it is bad… but…see earlier concerns.
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3.5mm, and 2.5mm connectors can be of good quality. The OEM jack as well if attention is given to that component. I had a custom retermination of a Cardas Clear done for a set of Focal Clears with dual 3.5mm and was terminated on the amp end with 2.5mm balanced. Cardas did an excellent job on fitting the termination and making a great reterminated cable.
Bad connectors can come in all sizes.
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Better to pickup a short 3.5mm to 6.3mm cable. I’ve had a couple of adapters made by Arctic Cables in both 4pin XLR to 2.5mm and 4-pin XLR to 6.3mm. I like the short cable versus the metal adapter which can snap at the source.
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How well a headphone is driven by an amp is often governed by impedance matching. It depends on how Cambridge audio are running the headphone amp, if they are running it off the main amps with a circuit to change how it interacts with a higher impedance driver, or they might have a dedicated opamp circuit.
I have a dedicated headphone setup, they can be run off a raspberry pi (if you can get one!). And dedicated headphone amps do not need to be expensive. Schiit, iFi, Topping, SMSL, Geshelli, JDS Labs, FiiO, all make affordable amps. Look over at Head-fi to see recommendations for the HD800.
I’d buy the HD800 from headphones.com with their warranty, if they don’t pair well with the Cambridge Audio, then you can try something else.
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Headphone output impedance is not very high. If you use a 250 ohm headphone or higher, it should be fine. The ones I’m currently using with my PC are 600 ohms.
Just my 2p there are loads above.
I am 95% headphones , using HD800 (not s) , I was using an Audiolab M-DAC and decided to upgrade. Eventually I settled for the Naim Uniti Atom HE and regret nothing.
I considered the EVO’s (150) being a long time CA user but the headphone 3.5 put me off and several reviews skirted around the sound output on phones , really hinting an issue. No one directly WOW’edit
I own a CXN v1 (so not Roon Ready) and my final option which I very nearly went with was the Sennheiser HDV800 . I had my HD800 renovated after a long search for new cans, I went to Sennheiser and compared HD800 and the s on the HDV800 it was stunning and started the itch for a new amp. I am a great believer in matching kit .
Finally the one box dedicated headphone amp swung the deal. The other bi element is ability to use balanced headphone output depending upon whether you believe it a benefit , its very difficult to compare as it requires a cable change to compare.
The bottom line is how much headphone listening you do , I do maybe 4-5 hrs a day, and virtually no speaker listening . I have no speakers so I go through my TV Soundbar just for background
Just another opinion but the HDV800 is a superb bit of German engineering !!
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This is simply not true.
The input of most headphone amplifiers is connected to the attenuator. If anything, you may want to set the volume of the pre-amp at maximum for the best signal-to-noise ratio. Unless that leaves too little volume adjustment on the headphone amp. I usually start with the output somewhere in the middle, and the input on the headphone amp at zero.
In the rarer cases where the volume control of the headphone amplifier is between gain stages (for one reason or another) then it is possible to overload the first gain stage if the pre-amplifier output is very high. Then it will just audibly clip. That’s easy to prevent by starting out with a lower output on the pre-amp.
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Yes, my integrated pre amp output ,outputs to my headphone amp, the attenuator is set at full volume on the headphone amp and I control the listening level using the integrated amp volume control and this is what was recommended by the headphone amp designer.
That was more a temporary solution to try out the headphones with his amp before he decided if he wanted to get a separate amp which would have a more robust connection ,ie. 6.35 or balanced but yes the strain on the socket would not have a good long term outcome.
My headphone amp to power supply cable is thick and heavy and I use supports underneath it to stop it straining on the sockets.
Each to their own.
There is a fundamental differance between a line out and pre out due to which side of the volume pot they are tapped.
If you want to play around balancing two volume pots then thats great. Personally i see no benefit to it.
Enjoy.