Amp recommendation and setup advice please

You should probably read this:

https://www.musicservertips.com/how-to-articles/iphone-digital-audio-output/

It appears that iPhone/iPod will not allow digital connection with some DACs.  You can try this with the Gustard, but honestly I think the best solution would be a laptop or desktop running media software such as JRiver.  JRiver has an excellent app (JRemote) that you can install on iPhone/iPad to remote control the JRiver on the computer (as long as you have a wireless computer network).  With JRemote you can view the music library in JRiver and play anything.

So, laptop/desktop is best solution (as I said you can always try the Lightning USB with Gustard).  Desktop will have a much better power supply and a better USB circuit.  Desktop computer is a LOT bigger and requires external monitor, but you can get a really good USB card to connect to the DAC, such as the SOTM, PPA (Paul Pang) or JCAT:

https://www.computeraudiophile.com/forums/topic/29199-would-you-recomemnd-an-audiophile-usb-card/

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I thought of another thing.  If you really wanted to use XLR cable for both DAC and phono preamp, you would have to get a separate preamp and amplifier (instead of an integrated).  Some preamps will XLR inputs for two sources (though many will only have one or none).  This will drive the cost up a lot more and you will also need another XLR cable between preamp/amp.  I think we are already way over the $5,000 USD point with the system we are discussion.  Doing a separate preamp/amp could potentially push the cost up to the $8,000-9,000 point.  As you said, since you only "occasionally" listen to digital music, it may not be worth it.  The Wire World RCA cables for Gustard will still be very excellent (unless your phono preamp doesn't use XLR).

The SOTM would probably be best if you used the stock computer power supply, with the JCAT being second. The Paul Pang really needs an external linear power supply, but it is likely to be the best if you use a linear power supply.

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Please note that I am going to extremes here.  In your situation, I would just start out using a basic laptop for digital music to the Gustard -- you may be perfectly happy with this.  If you feel this sounds "weak" or "grainy" or "dirty", you can always upgrade to a desktop and/or USB card in the future.

@laher – Some more information.  PS Audio is discontinuing the Nuwave Phono Premp and selling it at a big discount (about 48% off) for $999 USD.  If you are interested, you could contact them to see about shipping to UK and to make sure it is configured internally to support your local AC voltage (either 110V or 220V):

http://www.psaudio.com/products/nuwave-phono-converter/

A good review here:

http://www.theabsolutesound.com/articles/ps-audio-nuwave-phono-converter/?page=3

It appears that the PS audio Phono is highly configurable to match up with a lot of different turntables (maybe not all of them).  Another feature is that it has an analog-to-digital converter.  This allows you to connect it to your DAC to play the audio.  I have heard this is a very detailed sound, but maybe not quite as “analog”.  It’s just another option for you to work with.

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This was an email I sent to another friend advising on a turntable RCA cable.  He actually bought the Equinox and says it sounds excellent as a turntable cable.

https://forum.audiogon.com/discussions/man-this-is-frustrating

teo_audio has some very interesting thoughts on turntable cable burn-in. He says that turntable cable never burns in because the voltage levels never reach proper heights. Interesting.

Almarg also says that that it’s critical to have as low as capacitance as possible in a phono/tonearm cable. I remember reading once a long time ago that phono cables should be very small gauge (like 28awg). This could be why.

Based on that, it seems to me that one cable I would consider for this would be the Wireworld Equinox 7 RCA. It uses very small 31awg OCC copper strands. It is not quite solid-core. The strands are grouped into collections of 5 strands, which are all side-by-side (like a ribbon cable). It’s not a fully stranded cable, but it’s not fully solid-core either. It uses 10 strands for each conductor, making it a 21awg cable. The small strands give you the low capacitance (good for tonearm) and the collection for 21awg lowers the resistance and would open up the sound.  It’s important to note that the more expensive Eclipse 7 cable would not work as well because the copper strands are little bit larger 29awg.