I feel I may be covering old ground here, but is there anything actually worth considering around the £400 Mark than the MoJo?
It's mainly for desktop use. The only other possible option is the Cambridge Audio DacMagic 200m, but it has a load of stuff I don't need (Bluetooth etc).
Anything else out there worth considering?
For £400 there are tons for options out there, this is one of my top picks for the price,
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This will give you completely transparent sound and power your 660s amazingly well, lot of people including myself use this as a 'basic reference setup', I've ran the HD650's on this and it beats many amps way above it's price range, also don't worry about it coming from the states, the support from JDS Labs is first rate, email response is very quick and quite often the founder himself answers them, I recently had a JDS Labs Element 2 that was faulty and even though I wasn't the original owner they offered to send out a free replacement express delivery from the states!
If you want to grab something local (UK) value chi-fi then here are a few options for you,
Fosi Audio SK02 £84
Fiio K11 £119 or R2R version around £149 (R2R isn't inherently better, it's a preference)
Xduoo XA-02 £199
Don't be fooled by the low price, all of these will power the 660s like a champ!
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Something a bit more expensive,
Hifiman EF499 £299
SMSL DO400 £429
Fiio K9 £475 (if you google can get £374.99
open box) <---- This is a bargain!!! for this price this would probably be my top choice.
Xduoo TA-22 DAC/Tube £489
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Many more options out there!
You mentioned the Mojo, (I'm assuming Mojo 2), this has a special place in my heart, right now you can grab one from Peter Tyson for £305.10 VIP price (you just need to be subbed to emails), this is arguably imo the best sounding dac/amp combo under a grand, it's genuinely phenomenal, but unfortunately it isn't without it's foibles, Micro-USB charging/power, non-intuitive menu system, small, no rotary volume dial, your limited to 600mw into 30 ohms, although it powers my HD650's very well.
The pluses are that is does have battery protection if you want to use as a desktop amp/dac and powered on all the time, lossless EQ, it's portable which can be very handy, and also the two 3.5mm outputs can be used as dual lineouts, this is great if you want to use the Mojo as a dedicated DAC pushing audio to a headphone amp and a separate stereo integrated amp for speakers.